Comparison of the Kinetics of Colony Forming Units in Spleen (CFUs) and Culture (CFUc)

S ummary . A comparative study of the in vitro kinetics of CFU s and CFU c indicated that: (a) there is a five‐ to seven‐fold increase in CFU c numbers under conditions which result in a progressive depletion of CFU s ; (b) refeeding the cultures delayed the late decline in CFU c numbers without affecting CFU s ; (c) using cycling CFU s or adding erythrocytes to the culture markedly improved CFU s survival without affecting that of CFU c ; (d) the CFU c /CFU s ratio changes during marrow regeneration, but in the opposite direction which would be expected if CFU c were a part of the cycling CFU s population. It is concluded from these that the two operational definitions, CFU s and CFU c , do not denote the same cell type under the culture conditions described.