A bioassay system for two types of colony stimulating factor in human serum.

The biological assay of factors with granulocyte-macrophage colony stimulating activity in human serum poses special problems. Assays based on colony counts suggest, often erroneously, that serum lacks colony stimulating factors (CSF) but interacts with other materials to potentiate or produce factors. Although the scoring of total clone numbers reveals the presence of CSF in serum, this is still no sufficient for the interpretation of the effects of mixing materials containing CSF, because increments in clone numbers are not directly proportional to increments in CSF. This is particularly important in serum assays because two types of activity are present, one which stimulates progenitor cells directly, and another which results from the interation of serum and bone marrow adherent CSF-producing) cells or peripheral blood leucocytes, indicating the presence of both direct acting and "adherent cell dependent" CSF. A biological assay is described which uses analysis of dose-response curves of clone formation in agar culture, and allows simultaneous assay of both types of activity. The criteria for the selection of suitable target progenitor cell populations are discussed.