Granulocyte-macrophage colony forming units (CFU-GM) were studied in cultures of bone marrow from 16 apparently healthy normal controls, 9 patients with the myelodysplastic syndrome, 5 patients with myeloproliferative disease and 2 with myeloma. Supernatants from non-stimulated 72 hr cultures of nonadherent mononuclear blood cells ("lymphocytes") stimulated the forming of an average of 38.4 colonies per 100,000 cells from normal marrow. The addition of GIBCO's commercial conditioned medium or of a medium produced by lymphocytes stimulated with different concentrations (5, 10 and 20 mcg/ml) of an acid lysate of thymus (thymomoduline), increased growth to 65.2 - 55.4 colonies (p less than 0.001 to 0.05). Similarly, a significant increase (p less than 0.05) was found in the number of clusters and colonies formed in cultures of marrow from patients with the myelodysplastic syndrome. In contrast, no growth was found when the thymus acid lysate was added directly to the bone marrow cultures, suggesting that the lysate induces the production of colony stimulating activity by lymphocytes, but does not contain it. Similarly no significant increase was found as regards the initially high number of colonies from the five patients with myeloproliferative disease, or as regards the initially low number in the two myeloma patients.