Thymic myoid cells produced macrophage lineage cell stimulatory factors. Activities were separated into two factors on DEAE-Sepharose CL-6B chromatography: one eluted at lower concentrations of NaCl and the other at higher concentrations of NaCl. The latter fraction was purified to homogeneity with an apparent molecular weight of 100,000. This factor stimulated the growth of macrophage-lineage cells from the bone marrow, but not that of granulocytes, megakaryocytes or erythroblasts. The 100,000 MW factor was able to induce Ia antigens on proliferating bone marrow cells. These results suggest that myoid cell-derived 100,000 MW factor plays significant roles in the generation of Ia-positive macrophage lineage cells which are important for T-cell development in the thymus.