The proliferation of CFUs as measured by the 3H-thymidine inactivation test in organ cultures of mouse embryonic liver is maintained for 3-4 weeks. As regards the proliferation rate, CFUs located in the upper washed layers differ from those located in the deep culture layers. CFUs from washings manifested significant proliferation only in individual experiments and in the early times (3-9 days), while in the rest cases it remained at the zero level or was insignificant. CFUs from the deep layers rapidly proliferated for 3-4 weeks. Thus, the recovery of the content of CFUs and other hemopoietic cells in the washed layers is effected at the expense of proliferation of hemopoietic stem cells in the deep culture layers. After proliferation of CFUs ceases, their number in the cultures diminishes.