Brief report: qualitative differences in the biological activity of two sources of colony stimulating factor: GCT conditioned medium and leucocyte feeder layers.

A most stringent test for the non-identity of biologically active materials is provided by the techniques used for parallel line type bioassay of these materials. We have studied commercially available GCT conditioned medium using a parallel line type biological assay for granulocyte-macrophage colony stimulating factors (GM-CSF) and mononuclear leucocytes as a reference source of GM-CSF. Only test and reference GM-CSF sources which have active constituents with identical dose response characteristics can produce parallel line displacement in this assay system. GCT conditioned medium failed to produce parallel displacements in five different assays. We must therefore conclude that one or more active factors from the test and reference sources of GM-CSF (GCT conditioned medium and mononuclear leucocytes) are not identical. This indicates a need for caution when substituting GCT conditioned medium for leucocyte feeder layers in human bone marrow cultures.