A study of maternal blood samples from 280,000 pregnancies in an 8-year period has shown 38 examples of anti-C (without anti-D) sensitization. This frequency (0.14/1000 pregnancies) was lower than that previously found for anti-c (0.63/1000 pregnancies) and for anti-D (2.55/1000 pregnancies). Although most of the 38 babies born to mothers with anti-C were unaffected by haemolytic disease of the newborn and none was anaemic at birth, two required exchange transfusion for hyperbilirubinaemia and one a top-up transfusion. Five cord blood samples had a positive direct antiglobulin test.