In 5 patients of phenotype ccDEe or ccDEE, the transfusion of 2-14 U of C-positive blood was followed 5-9 days later by haemoglobinuria lasting 2-4 days. Anti-C was the only antibody present in all 5 cases but was always of low titre; with CC cells the maximum titre recorded with the indirect antiglobulin test was 8 and with the agglutination of enzymetreated cells, 128. The discordance between the weakness of the antibody in vivo and the amount of haemoglobin released intravascularly is surprising but may be related to the type of IgG molecule of which anti-C is composed.