Two distinct types of colony-stimulating factor (CSF) have recently been described in human tissues and cultured cell lines. Antisera to purified type I and II CSF were prepared in rabbits. Anti-CSF I antibody inhibits CSF I, but has no effect on CSF II. It cross-inhibits CSF I from several other human sources, but does not inhibit CSF from mouse lung or mouse L cells. Anti-CSF II antibody inhibits the activity of CSF II, but has no effect on CSF I. A radioimmunoassay for CSF I has been established. Competitive binding assay further demonstrated the immunological differences between CSF I and II. When CSF II is used to stimulate human marrow cells fractionated by sedimentation velocity, two populations of CFU-C are separated, one sedimenting at 8 mm/h and forming colonies by day 7, and a second sedimenting at 6.8 mm/h and forming colonies by day 13. In contrast, CSF I does not stimulate colony growth by day 7 but does do so by day 13 in cells sedimenting between 7.2-8.5 mm/h. These results indicate that CSF I and II are distinct in their biochemical, immunological, and functional properties.
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