A simple method for quantitating endogenous CSA

The enhanced colony-formation by human bone marrow cells in vitro, produced by the addition of lysed rat red blood cells, has been shown to be due to an increase in the sensitivity of the colony-forming cells (CFUc) to colony-stimulating activity (CSA). Rat red cell lysate does not contain CSA, nor does it increase CSA production by cells. The sensitivity of colong-forming cells is sufficiently increased by the lysate to give colony-formation in the absence of any added source of CSA. Consequently, it is possible to assay for locally produced (endogenous) CSA without the need for cell separation and the complications this entails. Furthermore, the linear relationship between the numbers of cells plated and the colony yield simplifies the quantitative interpretation of the results.