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In vitro dose-response curves of unstable chromosome aberrations in human lymphocytes have been obtained for 250 kV X-rays and cobalt-60gamma-radiation. The aberration yields have been fitted to the quadratic function Y = alphaD +betaD2, which is consistent with the single-track and two-track model for aberration formation. The values of the coefficients alpha and beta support the hypothesis that the dose-rate effect is limited to the D2 term. The main difference between the coefficients for X- and gamma-radiation is in the alpha values, indicating that X-rays are slightly more efficient, at lower doses, in producing two lesions with a single ionizing track. The lower limits of dose estimate, with 500 cells analysed, are 4 rad for X-rays and 10 rad gamma-radiation. Further evidence is presented confirming that, for cytogenetic dosimetry, in vitro dose-response curves should be prepared by irradiating whole blood maintained at 37 degrees C and prior to PHA stimulation. Curves were plotted showing the variation of the number of cells without aberrations with radiation dose and the shape of these curves were compared with those from human cell survival experiments.

The induction of unstable chromosome aberrations in human peripheral blood lymphocytes exposed in vitro to protracted doses of cobalt-60 radiation is presented. Four dose response curves have been produced using constant exposure times of 1,3,6, and 12 h. The data fit well to the linear quadratic model and the yield coefficients have been compared with those obtained for acute (less than 10 min) exposure. The quadratic coefficient is dependent on irradiation time and decreases approximately as predicted by Lea and Catcheside's G-function hypothesis. The possibility of a small proportion of much longer lived breaks is discussed. For purposes of biological dosimetry it is sufficient to assume a mean time of 2 h and a single exponential function for the repair of lesions when relating the effects of brief and protracted exposure.

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Although the developmental switch from fetal to adult haemoglobin production has been well characterized in terms of protein synthesis, very little is known about its control at the cellular level. In order to determine whether the switch is controlled by environmental factors or programmed into the haemopoietic cell population, we have studied the effects of transplantation of fetal and adult haemopoietic cells into lethally irradiated lambs on haemoglobin synthesis by the transplanted cells. In two lambs which were irradiated but not transplanted, two lambs which were grafted with autologous marrow, and one lamb which was grafted from an allogenic twin, haemoglobin synthesis showed an adult pattern following transplantation, with gamma-chain synthesis never exceeding 7% of non-alpha chain production. Of 11 lambs transplanted with fetal haemopoietic cells, only two showed evidence of engraftment. During the 24-26 d that these animals survived, haemoglobin synthesis showed a predominantly fetal pattern, though there was a gradual increase in beta-chain production of donor origin. This increase occurred earlier than would be expected from the gestational age of the transplanted cells, but more slowly than might be expected if environmental factors were entirely responsible for expression of adult or fetal haemoglobin synthesis.

The late effects of irradiation with single and fractionated doses of X rays (250 kV) and fast neutrons (42 MeVd----Bc), on the cutaneous and subcutaneous tissues of the pig, have been evaluated from measurements of changes in relative field length. These were determined at intervals of 26-104 weeks after irradiation. For fractionated irradiation with X rays the average fractions exponent, N, obtained from a log-log plot of iso-effect dose (ED50) against fraction number was 0.41. This was independent of the period of assessment, with no significant indication of a time factor. However, the exponent N did vary with the level of effect and was in the range 0.33-0.51. It was greatest for a greater than or equal to 10% reduction in relative field length. Assuming the validity of the linear quadratic model of cell survival, the alpha/beta ratio was 1.95 Gy. However, this model fitted the data less well for the least severe levels of damage, and for these the alpha/beta ratios were not significantly different from zero. Irradiation with fast neutrons showed a small effect of fractionation for doses given in greater than or equal to 6 fractions compared with a single dose. There was no significant increase in iso-effect dose when the dose was given in 30 fractions compared with 6 fractions. The relative biological effectiveness for late cutaneous and subcutaneous damage for the energy of fast neutrons used did not vary with the period of assessment, i.e. 26-52 weeks compared with 65-104 weeks, and was not significantly different from that previously obtained for ischaemic dermal necrosis, seen after higher doses, at 12-20 weeks after irradiation.