Pig membrane cofactor protein (MCP; CD46) is a 50 000–60 000 MW glycoprotein that is expressed on a wide variety of cells, including erythrocytes. Pig MCP has cofactor activity for factor I‐mediated cleavage of C3b and is an efficient regulator of the classical and alternative pathway of human and pig complement. A panel of 10 monoclonal antibodies (mAbs) was collected from two different laboratories; all of these mAbs were raised against pig leucocytes and all recognized the same complex banding pattern on sodium dodecyl sulphate–polyacrylamide gel electrophoresis (SDS–PAGE) of erythrocyte membranes. All were shown to be reactive with pig MCP and were divided into four groups of mutually competitive antibodies based on competition studies for membrane‐bound MCP and for soluble MCP, the latter by surface plasmon resonance (SPR) analysis. The antigenic properties of membrane‐bound and soluble MCP were similar, although some interesting differences were revealed. None of the 10 mAbs were cross‐reactive with human MCP and only one showed cross‐reactivity with leucocytes from a panel of large mammals – a weak cross‐reactivity with a subset of dog leucocytes. All antibodies in one of the epitope groups and some in a second epitope group were able to block the functional activity of pig MCP, as measured by inhibition of MCP‐catalysed C3 degradation by factor I.