Addition of irradiated bone marrow feeder from guinea pigs and rabbits to cultures of passed rabbit bone marrow fibroblasts considerably decreases the efficiency of colony-formation and the total amount of fibroblasts in cultures. The presence of irradiated rabbit blood platelets sharply increases these parameters. The addition of irradiated bone marrow feeder to rabbit bone marrow fibroblasts cultured under conditions promoting cell differentiation also decreases the total content of fibroblasts in cultures. Our results suggest that cultured descendants of rabbit bone marrow stromal precursors preserve both the sensitivity to inhibitory factors produced by bone marrow cells coexisting with stromal precursors in cultures (typical of stromal precursors of these animals) and high sensitivity to growth-stimulating platelet-derived factors (typical of stromal precursors from rabbits and other animal species).