The distribution of spleen colony diameters was determined 5.5, 8.0, 10.5 and 13.0 days after injection of normal bone marrow cells to lethally irradiated recipients. A relative lack of small colonies on day 8.0, as compared with days 5.5, 10.5 and 13.0, argued against a time continuum in colony appearance. The spleen colonies observed after 10 days or more probably represented a mixture of colonies which developed from the originally transplanted CFU-S and those arising from secondary CFU-S. Thus, late appearing spleen colonies may not necessarily identify a different, less mature, population of CFU-S. Administration of increasing amounts of bone marrow cells was used for comparing the linearity of the CFU-S assay for colonies observed after 8 days or after 12 to 13 days. The influence of overlapping colonies on the results was considerably augmented if large spleen colonies were observed after 12 or 13 days. Subsequently the CFU-S assay lost much of its quantitative character. We believe that some previously published data might have been misinterpreted by neglecting the important differences between 'early' and 'late' CFU-S assays.

Department of Pathophysiology Faculty of General Medicine Charles University Prague Czechoslovakia

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