Variations in the concentration and physical characteristics of the bone marrow derived colony forming cell(CFC) have been studied in patients with acute leukaemia. Two-hundred-and-fifteen marrow samples from 83 patients provide the basis for this analysis. CFC concentration confirmed the clinical remission/relapse status and yielded some guidelines to prognosis in individual patients while the proportions of CFC in DNA synthesis also proved to be a most reliable indicator of disease status. In remission, CFC concentrations return to normal values whilst on presentation and in the relapse phase of acute leukaemia CFC numbers are reduced. Biophysical profiles of CFC established using albumin density gradient and velocity sedimentation studies also indicated the state of the leukaemic process in individual patients. By applying physical laws to the data obtained from such profiles, the mean volume, diameter, density and mass of CFC were calculated. CFC from leukaemic patients in relapse were up to twice the volume and mass although slightly less dense than CFC from normal patients. The reasons for these changes are explained and discussed.