This report demonstrates that complement and classical complement component activities are detectable by standard haemolytic methods in foetal porcine serum as early as the 40th day of the 115–120 gestational period. At this stage of development only miniscule amounts of immunoglobulins are detectable in the serum. Complement activity equal to that of the adult pig is present in the serum of the piglet at 1 week of age. Pig colostrum, from which lipids were removed, contained large amounts of haemolytic C′1, C′2 and classical C′3 as well as C′1q protein and C′1q agglutinating activity. It is of major interest that the activities of C′1q, haemolytic C′1 and C′2 are five to thirty times higher in suckling than in age matched non-suckling piglets. These observations suggest that biologically active complement proteins are transferred from the colostrum through the intestine to the piglet protein pool.