Production of colony stimulating factor in long‐term bone marrow cultures

Previous studies have shown no detectable colony-stimulating factor (CSF) in media harvested from long-term bone marrow cultures. In the present experiments supernatants from long-term cultures established in three laboratories were assayed for CSF by colony assay and by radioimmunoassay (RIA). Most samples were devoid of biologic activity but all contained CSF as judged by RIA. Biologic activity was found in the majority of samples after diafiltration to remove low molecular weight inhibitors or 5-fold concentration by ultrafiltration. Samples that remained inactive in the colony assay were subjected to gel filtration on Sephadex G-150 to remove potential high molecular weight inhibitors. Biologic activity remained lower than that by RIA in two of three samples tested. Thus, most long-term cultures appear to contain biologically active CSF but this activity is masked by various types of inhibitors. In addition some media appear to contain material that is only detected by RIA.