M embrane cofactor protein (MCP) of the complement system is a iC3/C3b binding molecule with cofactor activity that has been identified on all human peripheral blood cells except erythrocytes. Human mononuclear and platelet MCP is dimeric with molecular weights of 68,000 and 63,000 and is expressed in three phenotypic patterns. To further determine its tissue distribution, surface-labeled human fibroblast, epithelial, and endothelial cells and cell lines were assessed for the presence of MCP by iC3 affinity chromatography and by immunoprecipitation with a monospecific anti-MCP rabbit polyclonal antibody. All sources of adult and fetal fibroblast and epithelial cells and cell lines examined and umbilical vein endothelial cells expressed MCP. The molecular weight and phenotypic patterns of MCP were similar to those of peripheral blood cells. MCP was synthesized by fibroblast and epithelial cell lines. Solubilized extracts of these cell lines expressed factor I-dependent cofactor activity for the first cleavage of iC3/C3b which was abrogated by removal of MCP. Expression of MCP was modulated by SV40 transformation of two fetal fibroblast lines. There was a 5- to 10-fold increase in expression of MCP and a preferential expression of the lower species such that the phenotypic designation was changed. The wide tissue distribution and activity profile of MCP suggest that it is likely to play an important role in the regulation of the complement cascade.