The effects of testosterone and erythropoietin (ESF) on erythroid colony formation in normal human bone marrow cultures were studied in vitro using a methyl cellulose gel system. Testosterone was found to produce a significant increase in erythroid colony formation at concentrations of 10–4–10-4M in vitro. In this system, the numbers of erythroid colonies formed per plate increased in direct proportion to the increase in the number of erythroid precursors inoculated as well as to the increase in the dose of ESF in vitro. In addition, a synergistic effect of a combination of testosterone and ESF on erythroid colony formation was seen when ESF was present at high concentrations. These data suggest that a greater number of erythropoietin-responsive cells are available for ESF to differentiate into the nucleated erythroid cell line in the presence of testosterone, indicating that the effect of a combination of testosterone and ESF is greater in enhancing erythropoiesis than the additive effects of either agent alone.