Origin and Clinical DescriptionThe first affected animals were found among the offspring of closely related pairs in the CBA/CaH inbred strain. The animals were first noticed to be losing weight, at the age of a few months. They became very thin and hunched, and their urine became colourless. At necropsy the kidneys were pale, contracted, and granular.At first, mice were classified as affected or not on the basis of the colour of the urine and the macroscopical appearance of the kidneys. Every two weeks from about 3j months of age the colour of a urine specimen from each mouse was noted. After three consecutive specimens had been colourless, or when the mouse had reached the age of 6 months, whichever was the earlier, it was killed and the kidneys were inspected.Later, this method was replaced by tests of urinary protein using Hema-Combistix reagent strips (Ames). Proteinuria is normal in mice, the quantity varying with a number of factors including diet (Bernstein, 1966) and urine from normal CBA/CaH mice gave a Hema-

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As in the previous investigation a dose as low as IOO R. caused a marked reduction in the four types of red-cell precursors. Similarly the fall in numbers occurred very soon after irradiation, reductions being seen at the earliest time interval at which observations were made in the present experiments, i.e. at 4 hours after 400 R. At each dose level the more mature non-dividing cells (late and intermediate normoblasts) were again found to be proportionally 365 * Postal address: M.R.C. Radiobiological Research Unit, Harwell, Didcot, Berks. 366 E. V. Hulse Recovery ofErythropoiesis after Irradiation 367 400 R. a series of values in the region of normal or slightly above were followed at 42-56 days by three successive values of about two-thirds of normal. The 70 day value was in the control range. S~H , B. and HERDAN, G. (1954). 'A quantitative study ofthe effects of compound E, compound F and compound A, upon the bone marrow of the guinea-pig.'J. Amit. (Lond.), 88, 115.

Energy Research ~s~a b l~s~m e~t , H a r u d , BerkshirrIONIZING radiations produce a profound effect upon the bone marrow but thc extent of thc damage is difficult to assess because both the cellularity of the marrow and the relative proportions of the various cell types alter simultaneously. Previous workers have used histological methods (Bloom, 1948) or the examination of imprints of the bone inarrow (Roscnthal, Pickering and Goldschmidt, 1951 ; Rosenthal, 1955), but although these methods give estimates of the cellularity neither gives truly quantitative results. An attcnipt has been made to study the damage produced by various doses of whole-body X-irradiation more precisely by using a technique based on that of Yoffey, Ancill, Holt, Owcn-Smith and Herdan (1954) which is capable of giving the absolute number of cells pcr cmm. of each cell typc present. The results relating to the effects on the red-cell precursors for the first 48 hours after irradiation are given here. METHODMale rats of an inbred albino strain aged between 4 and 7 months were used in groups of litter inatcs. The animals in each litter were randomly allotted differcnt doses of wholebody X-irradiation, or different time-intervals following the samc dose of X-rays, and at least onc sib was used as a non-irradiated control.One series of animals was examined 24 hours after irradiation with doses of 2s r, 50 r, 75 r, IOO r, zoo r, 400 r, 600 r, 800 r, 1000 r, 2000 r and 5000 r. The LD,,/30 days for the strain is about 600 r.Three series of animals were used to study tlic rate of development of irradiation dainagc and were killed from 2 hours to 48 hours after IOO r, 600 r and 5000 r. Either three or four aninials were examined at each dose or time-interval, except at 24 hours after 25 r when six aninials were used.The rats werc irradiated either singly or in pairs in aluminium boxes measuring 25 cni. x 20 cm. x 10 cm. Thc radiation factors were 240 kv, 43 r/min.; 250 kV, 48.8 r/niin.; or 250