CD59 (protectin) and CD46 (membrane cofactor protein, MCP) are membrane-bound complement regulator proteins which inhibit complement-mediated cytolysis of autologous cells. CD59, a phosphatidyl-inositol-anchored glycoprotein, inhibits the formation of the terminal membrane attack complex (MAC) of complement and was found to be a second ligand for CD2 contributing to T-cell activation. In 20 colorectal normal mucosa samples, in ten adenomas, 71 carcinomas and in ten liver metastases derived thereof, CD59 was inconsistently expressed in the epithelial compartment. In carcinomas CD59 expression in the whole neoplastic compartment was more often found in well- and moderately differentiated tumours. By contrast, focal expression or even complete lack of CD59 was more often found in poorly differentiated tumours (P = 0.021). In addition, carcinomas without metastases at the time of operation (Dukes A/B) more often expressed CD59 in the entire neoplastic population compared to those carcinomas which had already metastasised (P = 0.018). There was no correlation between the mode of CD59 expression in colorectal carcinomas and the tumour type or location. CD46 has C3b/C4b binding and factor-I dependent cofactor activity and is broadly expressed in various cells and tissues. In the epithelial compartment of normal colorectal mucosa, of all adenomas, carcinomas and their liver metastases, CD46 was expressed throughout the epithelial compartment. Since CD46 was consistently expressed in colorectal carcinomas the low expression or even lack of CD59 in a subset of tumours might not lead to critical complement-mediated attack of CD59-negative tumour cells. Regarding CD59 as a natural T-cell ligand involved in cognate T-cell-target-cell interaction, however, loss of CD59 might well be a selection advantage, provided that tumour antigen-mediated T-cell toxicity in colorectal carcinoma exists.