The sera of 96 consecutive Rh-negative female infants born to Rh-positive mothers were examined at birth, and sera from 88 of these infants were examined for the presence of Rh-antibodies at the ages of about three and eight months. A two-stage papain test and an AutoAnalyzer method were used for antibody screening and identification. Weak anti-D antibodies were found by the papain and AutoAnalyzer techniques in two cord sera, In neither case could the antibodies be demonstrated in samples taken on later occasions. Weak anti-D antibodies were found by the AutoAnalyzer technique but not by the manual methods in the sera of two other infants at the age of eight months. These antibodies could still be demonstrated by the same technique in samples taken about one month later. Though far from conclusively, the results support the "grandmother theory", but because of the low incidence of sensitization and uncertain nature of the anti-D antibodies demonstrable only by the AutoAnalyzer technique, anti-D prophylaxis is not recommended for newborn Rh-negative female infants with Rh-positive mothers.