Studies were conducted with diffusion chambers (DC) filled with cell suspensions from different CF1 murine hematopoietic tissues: adult peripheral blood; adult tibial marrow; day 17-5 of gestation fetal liver, spleen and thymus; day 14-5 gestation fetal liver; day 10-5 of gestation yolk sac. After an initial decrease in DC cell numbers on day 2 of culture, growth of each cell group continued, but, at different rates. ATM had the highest growth ratio and FT-D17-5(2) had the lowest. The growth rates for APB and FL-D17-5 were similar. FS-D17-5 and FL-D14-5 cultures did not recover from the day 2 values (i.e. FL-D14-5 DC values on day 13-14 of culture were half that recorded on day 2). The YS-D10-5 DC cell numbers continued to increase throughout the 14 days of study. The profile of cellular elements from the DCs did not reflect the original cell suspensions. The predominant cell type recovered from peripheral blood cultured for 14 days was the macrophage. By day 10-14 of culture, the populations of cells harvested from the fetal tissue DC groups were similar to that of tibial marrow. Both proliferative and mature granulocytes, and macrophages were the predominant cell types. The yolk-sac pattern of cytodifferentiation recorded on day 7-14 was unlike that of the other groups. These DC cultures were comprised of mainly macrophages and plasma cells.