Humoral stimulatory activity (HSA) is detected in the sera of neutropenic rodents and humans by its effects on in vitro marrow cell [3H] TdR incorporation. Increased levels of CSA are also found in such sera; however, the relationship between these two activities has not been established. To examine this question, sera were obtained from mice, rats and humans at the peak occurrence of HSA following cyclophosphamide administration and were incubated with rabbit antiserum directed against mouse L-cell CSF. Anti-CSF neutralized the HSA effect on species-specific marrow cell [3H] TdR incorporation relative to the effects of normal sera with an efficiency of anti-CSF produced graded neutralization of the human HSA effect on human marrow cell proliferation. This neutralizing effect of anti-CSF on human HSA was obrogated by prior incubation of anti-CSF with sheep-anti-rabbit-IgG (S-anti-R-IgG) serum. The human serum activities on marrow cell proliferation, as determined by the [3H] TdR incorporation assay, correlated closely with their effects on human granulocyte colony formation in agar culture. These data indicate that human and rodent drug-induced HSA are cross-reactive with CSF as measured by a simple proliferation assay.