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The Ca2+-Mg2+ adenosine-5'-triphosphatase (ATPase) in sarcoplasmic reticulum has been covalently labeled with the phosphorescent triplet probe erythrosinyl 5-isothiocyanate. The rotational diffusion of the protein in the membrane at 25 degrees C was examined by measuring the time dependence of the phosphorescence emission anisotropy. Detailed analysis of both the total emission S(t) = Iv(t) + 2IH(t) and anisotropy R(t) = [Iv(t) - IH(t)]/[Iv(t) + 2IH(t)] curves shows the presence of multiple components. The latter is incompatible with a simple model of protein movement. The experimental data are consistent with a model in which the sum of four exponential components defines the phosphorescence decay. The anisotropy decay corresponds to a model in which the phosphor itself or a small phosphor-bearing segment reorients on a sub-microsecond time scale about an axis attached to a larger segment, which in turn reorients on a time scale of a few microseconds about an axis fixed in the frame of the ATPase. A fraction of the protein molecules rotate on a time scale of 100-200 microseconds about the normal to the bilayer, while the rest are rotationally stationary, at least on a sub-millisecond time scale.

The numbers of CFU-S which developed in spleen colonies were measured 11 days after injection of irradiated mice with marrow from normal mice or mice which had been treated in one of a variety of ways. The broad spread of CFU-S numbers, seen by other authors, in colonies derived from normal marrow was confirmed. However, the range and distribution of CFU-S per colony was generally different in colonies derived from the marrow of mice which were recovering or had recovered from some form of depopulation. From the data obtained, the mean CFU-S/colony, M1, and the probability of self-renewal, p, of the CFU-S were calculated. These values are used to calculate the number of cell cycles undergone during development of the colony and, by making certain assumptions, the cell cycle time of the CFU-S. The plot of p against log M for the various samples measured should be linear if all CFU-S proliferate at the same rate in a growing colony. It is not linear, however, so that CFU-S obtained under different experimental conditions do not all undergo the same number of cycles. In general, treatments given to the mice result in a lowering of the capacity for self-renewal of their CFU-S and also to a shortening of their cell cycle time. Some of the possible implications of these findings are discussed.

A method is described for fitting a 'fraction labelled mitoses' curve to a set of data points and for estimating the values of the best fitting parameters of the cell cycle. Estimates of the SE of the parameters are obtained. The method depends on the fact that when gamma distributions are used t o describe the durations of the phases ofthe cell cycle, the Laplace transform of a FLM curve can be described by simple analytic functions enabling a least squares fit to be made to a set of Laplace transforms of the experimental data. The method is easy to program and quick to execute. I N T R O D U C T I O NThe method of labelled mitosesis one of the most important methods for studying and estimating the inter-mitotic intervals and the length and position ofthe S period (Quastler &Sherman, 1959). Although it is easy to get approximate values for the cell cycle parameters from the fraction labelled mitoses (FLM) curve, there is no graphical method which has any degree of precision. The calculation of a FLM curve from a set of values of the parameters is not simple. Methods have been described by Barrett (1966), Takahashi (1968), Bronk (1969 and Trucco & Brockwell(l968). These use Monte Carlo methods or divide the cell cycle into a large number of random compartments and solve a set of linear differential equations.These methods do not lend themselves readily to the estimation of the best values for the parameters although this has been attempted by Barrett (1970) and by Steel & Hanes (1971), but their methods lack estimates of the standard errors (SE) of the parameters. Hartmann & Pedersen (1970) obtain an explicit expression for the FLM curve which involves the summation of an infinite series but in practice only the first few terms need to be calculated. They use gaussian probability density functions but do not give estimates of the SE of the parameters. A similar solution has been described by Macdonald (1970). Trucco & Brockwell (1968) derive the Laplace transform of the FLM curve.

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