Membrane cofactor protein (MCP) is a cell-associated regulatory molecule for C system with C3b/C4b binding and factor I-dependent cofactor activity. mAb were raised against MCP and amounts and distribution examined on normal human cells and cell lines. The mean quantity of MCP was 3000 to 7000 copies/cell in normal blood cells, except for E which have no MCP. Of note, PMN did not fully reveal all MCP sites until incubated for greater than 30 min at 37 degrees C. In most tumor cell lines, except for B cell lineages, expression of MCP increased by 2- to 8-fold in comparison with the normal cell counterparts. Strikingly, recombinant granulocyte CSF treatment of myeloid cell lines and hemin treatment of an erythroblastoid cell line, K562, led to a decrease of MCP to near normal levels. In contrast, C3b/C4b receptor (CR1) tended to increase with granulocyte-CSF treatment in several cell lines. We simultaneously determined levels of decay-accelerating factor (DAF) and CR1 in these tumor cells, and tested susceptibility to C3 deposition via activation of the alternative C pathway. Of 21 cell lines we examined, 14 lacked CR1 and two lacked DAF; none, however, lacked MCP. A slight amount of C3 deposition was observed in some myeloid cell lines and EBV-infected B cell lines. However, C3 deposition did not reflect a defect in the regulatory proteins. Tumor cells bearing MCP, lacking CR1 or DAF, and undergoing no C3 deposition, may escape C attack due to the compensatory effect of MCP in the absence of the other regulatory proteins. High expression of MCP provides a convenient means for tumor cells to block C attack and survive in blood stream. We favor the interpretation that MCP is up-regulated in association with certain malignant disorders, and that cell differentiation results in a switch from an MCP-dominant state to a CR1-dominant state.