Amplification of colony stimulating activity in human serum by interaction with CSA from other sources

An enhancing interaction of a human serum factor with other sources of colony stimulating activity (CSA) is described. Using a mouse colony forming cell assay, the enhancing effect of human serum was observed with suboptimal doses of colony stimulating material from monocytic cell feeder layers, supernatant from peripheral monocytic cells, and L cell supernatant. The enhancing factor is heat stable, is not dialyzable and is not extractable by ether. The enhancing phenomenon does not require the presence of viable cells in the feeder layers. The observation of enhancing interaction makes interpretation of all data from CSA stimulation experiments more complex than previously appreciated.