An increase in triglyceride synthesis has been observed in cultures of human bone marrow fibroblasts after the cells reach confluence. The addition of hydrocortisone further enhances triglyceride synthesis. Conditioned medium from confluent cultures also stimulates adipogenesis, probably mediated by a factor released through a hydrocortisone-dependent process. Subcultures derived from confluent cultures grown in the presence of hydrocortisone show a decrease in replicative capacity, as measured by DNA synthesis. This effect may represent the onset of a more differentiated phenotype, which seems to correspond to a cell showing a buoyant density of 1.052 g/ml and a high rate of triglyceride synthesis.