I-J+ suppressor cells are probably involved in regulation of spleen colony formation by hematopoietic stem cells (CFUs).

Treatment of murine bone marrow and spleen cell suspensions with anti-I-Jk alloantiserum and complement resulted in an increased number of CFUs (revealed by Till and McCulloch exocolonization technique) compared with controls. The stimulatory effect was observed with 7-8 day CFUs and was negligible or absent with 11-12 day CFUs. Normal thymus cells treated with anti-I-Jk antiserum plus complement would augment the colony-forming potential of normal marrow cells.