Following 2 safe and successful trials in Rh-negative male volunteers, the 1st clinical trial to test the ability of Rh immunoglobulin to prevent Rh-im-munlzation of Rh-negative mothers at risk was initiated in April 1964 at the ColumbiarPresbyterian. Medical Center, New York, and subsequently at the Memorial Hospital of Long Beach (Calif). These 2 trial studies can be summarized as follows: of 471 Rh-negative mothers admitted to the clinical studies thus far, who were at risk of immunization by an Rh-positive fetus, 345 (174 protected with Rh-immunoglobulin and 171 controls) have been followed for periods of 6 months to 1 1/2 years after delivery; 20 of 171 control mothers but none of 174 protected mothers are actively immunized to RhQ. Of 21 protected mothers who have become pregnant again, all remain nonimmunized, and 15 have already delivered another Rh-positive unaffected infant. Six of the 1st 15 control mothers to deliver a subsequent Rh-positive baby were Rh-immunized and their infants were affected. Results of studies by others are equally promising.