Bone marrow cells in suspension were separated into a number of fractions on the basis of cell size by sedimentation at unit gravity through gradients of fetal calf serum. The colony forming units (CFU) from the various fractions were tested for their self-renewal capacity using a double transplantation technique. The results indicate that the CFU in the fractions containing slowly sedimenting cells have an increased capacity for self-renewal in comparison with CFU in fractions containing rapidly sedimenting cells. In addition, a culture method was used to select populations containing CFU with increased self-renewal capacity, and these CFU were shown to sediment slowly in comparison with CFU of lower self-renewal capacity obtained from control cultures. It may be concluded that at least part of the heterogeneity observed in the CFU content of individual spleen colonies arises from the composition of the initial cell suspension, probably from intrinsic differences between the stem cells themselves.