Some experimental facts show the interrelation between stromal fibroblast activity and hemopoiesis development in hemopoietic organs. We studied the activity of stromal fibroblast precursors in liver, spleen and bone marrow of 74 embryons and fetuses with 4-25 weeks gestation age. In 24 cases all these data have been studied simultaneously. In 4 week liver fibroblast precursors(CFUf) are not defined; at 5-16 weeks the number of CFUf was 5.2-3.6 /105 cells, at 25-27 weeks being 0.2/105 cells. The decrease of CFUf preceeded the decline of hemopoiesis. Before the start of hemopoiesis in the spleen /10 weeks/ we could register the maximal increase of CFUf number- 6,7/105 cells, which didn't change until 24 weeks of gestation. The highest CFUf activity was discovered In fetal bone marrow - up to 38.5/105 cells from 9-10 weeks to 14-16 weeks.This activity preceeded and correlated with high concentration of CFU g, BFUe, CFUgemm during 14-16weeks period. By 18-25 weeks this concentration was found to fall and approached to the data in postnatal period. We conclude that stromal microenvironment can affect the hemopoietic cell activity in fetal liver, spleen and bone marrow.