Red blood cells become polyagglutinable when the normally latent T-antigens of the red blood cell membrane are exposed. Unmasking of T-antigens results from removal of N-acetyl-neuraminic acid by neuraminidase, an enzyme commonly produced by a variety of bacteria. Red blood cells altered in this way are said to be T-activated. T-activated red blood cells can be agglutinated by anti-T, an antibody normally present in human serum, so that severe transfusion reactions may occur and have occurred, if T-antigen positive patients are transfused with normal whole blood or plasma. This can be avoided by transfusing only packed or washed red blood cells. From October 1978 to October 1980 we found T-activation in 16 pediatric surgical patients aged 3 days to 14 yr with severe anaerobic infections. This included patients with necrotizing enterocolitis, perforated appendicitis, megacolon, infected anal atresia and gas gangrene. The isolate neuraminidase-producing bacteria were Clostridium perfringens and Bacteroides fragilis. Clinical data of these 16 patients are briefly reviewed and the importance of T-antigen positivity for their management is discussed.