A SERUM that identifies a new blood-group antigen was obtained from the parturient mother of a baby with erythroblastosis fœtalis. Mrs. Kidd had had five previous pregnancies but no blood transfusions. No previous babies had erythroblastosis fœtalis. She was delivered on April 17, 1950, of a male infant who developed typical erythroblastosis fœtalis, and whose cells gave a positive direct Coombs's test. On routine testing of Mrs. Kidd's serum at that time, it was apparent that it contained a previously unidentified antibody.