Conditioned media (MCM) of cloned thymic myoid cells (IT45R92, R613Ad, and R615B2) were used to investigate their possible involvement in thymic biological events. Those myoid cells produced in a culture medium biological activities capable of stimulating the growth of thymocytes, spleen cells, and bone marrow cells of mice and rats. Surface markers detected on spleen cells proliferating in MCM were characteristic of monocyte-macrophage lineages (C3R, Fc gamma R, asialo GM1) and T-cell lineages (Thy 1) but not B cells (sIgG). Chromatographic studies also suggested that the biological activities of MCM could be separated into two different molecular entities, such as a colony-stimulating activity and an interleukin 1-like activity which supported the growth of monocyte-macrophage lineages and T-cell lineages, respectively. These results indicate that thymic myoid cells produce cytokines important for the regulation of intrathymic interleukin cascade by which clonally differentiated thymic lymphocytes may be expanded into a sizable pool.