The proportion of erythroid cells in mitosis was determined in direct preparations and in synchronized and unsynchronized short-term cultures from normal human bone marrow. The stimulating effect on the proportion of erythroid mitoses of bovine serum albumin (BSA), erythropoietin (EP) and human leucocyte conditioned medium (HLC) derived from stimulated lymphocytes, was also analyzed in short-term marrow cultures. In synchronized and unsynchronized cultures, the proportion of erythroid mitoses was markedly lower than in direct preparations, with a mean of 75% in the direct compared to 11% in the unsynchronized cultures and 7% in synchronized. Statistical analysis showed that EP had a substantial influence on the erythroid cells while HCL had a minor effect.