The content of stromal precursor cells in heterotopic splenic transplants from old and young mice changed appreciably after cross transplantation to old and young animals. The content of CFC-F in the young-->old transplants decreased almost 1.5 times in comparison with the young-->young transplants, the counts of CFC-F in old-->old transplants were minimum in comparison with all other groups (2.5+/-0.1), while in the old-->young group transplants this value increased almost 8-fold (to 19.0+/-1.3) and surpassed the control level. Age-associated shifts in the splenic stromal tissue were determined by regulatory influences of the host, rather than by decreased count of stromal precursor cells in the tissue.