The bone marrow content, proliferative potential and proliferative activity of precursor cells for stromal fibroblast colony forming cells (CFC-F) were investigated in young and old CBA mice. The relationship between CFC-F and the number of bone marrow nucleated cells and granulocytic-macrophagal precursors (GM-CFC) was studied as well. The results obtained showed increased CFC-F contents in old animals. The proliferative potential of old mice CFC-F did not appear to differ from that of young animals. The proliferative activity of bone marrow CFC-F and hemopoietic stem cells--spleen colony forming cells (CFC-S) was studied by determining the sensitivity to hydroxyurea administration. The responses were almost the same in young and old mice. A direct correlation between CFC-F and nucleated cells and GM-CFC precursors was found in young mice, but not in the old animals. The results of the present study have pointed to the reorganisation of the stromal tissue microenvironment in bone marrow in old age.