In 9 out of 26 newborns with necrotising enterocolitis (NEC) exposure of the Thomsen-cryptantigen (T-antigen), probably due to the action of circulating bacterial neuraminidase, was demonstrated on red blood cells. The serological titres seemed to correlate with the clinical course of the disease. Neuraminidase-producing clostridia were isolated in two of the patients. Reaction between the exposed T-antigen and anti-T-agglutinins, normally present in human blood, may lead to difficulties during blood transfusion. This potential transfusion hazard is best avoided by routine T-antigen-tests and by transfusion of packed or washed red blood cells to T-antigen-positive patients.