Since sera from most species contain colony-stimulating (CSF) and column-inhibitory factors, the net CSA [colony-stimulating activity] results from a balance between these opposing substances. Studies were performed to determine whether the increased CSA noted during neutropenia or endotoxemia [induced by Salmonella typhosa endotoxin] is due to an increase in CSF or to a decline in serum inhibitors. Rats were treated with cyclophosphamide, total body irradiation or endotoxin and bled during the peak CSA response. By separation studies using Sephadex G150, it was shown that serum levels of high MW inhibitors were unchanged from control values. Minimal CSA was detected in normal serum fractions; increased activity was found in fractions from neutropenic animals. During a post-cyclophosphamide rebound neutrophilia, serum CSA was undetectable; inhibitor levels were similar to those measured in untreated controls. The increased serum CSA seen with neotropenia is due to a true increase in CSF and not to a decline in circulating inhibitors.