A culture system was reproduced in which a marrow-derived adherent cell population, established in vitro, exerts a long-term promoting influence on proliferation of hemopoietic cells. Essential parameters of the system were investigated. The system was critically dependent on horse serum, and on the in vitro age of the adherent cell layer. The growth-promoting effect on hemopoietic cells seemed to be independent of the number of marrow cells per culture flask initially inoculated into the cultures to establish the adherent cell layer. In vitro established marrow-derived adherent cell layers from RFM (H-2f) and BALB/c (H-2d) mice could promote the long-term proliferation of syngeneic and allogeneic hemopoietic cells. Hemopoietic marrow cells from C3H (H-2k) could not be maintained on syngeneic or allogeneic (BALB/c, H-2d) adherent cell layers. Adherent cell layers of C3H (H-2k) could maintain hemopoietic cells of the H-2d (BALB/c) genotype. This culture system did not reproduce the in vivo phenomenon of allogeneic resistance. The relevance of these findings to the suggestion that the growth-promoting activity of adherent marrow cells on hemopoietic stem cells in vitro duplicates aspects of the in vitro hemopoietic microenvironment was discussed.