A retrospective analysis was made of the case records of 141 infants with jaundice due to rhesus incompatibility born in one unit over a six-year period during which phototherapy was introduced, in order to ascertain the effect of this form of treatment on the serum indirect bilirubin concentration. The infants were allocated to four groups of severity based on cord blood findings and the curve of bilirubin concentration drawn over the first 48 hours of life. Infants born before the advent of phototherapy acted as controls for those treated. There was a highly significant reduction in the level of indirect bilirubin reached in the mildest group not requiring exchange transfusion at birth, the mean bilirubin at 48 hours being lowered by 3.5 mg/100 ml in the treated group. Phototherapy exerted a lesser effect on the jaundice of more severely affected groups but still significantly reduced the rate of rise and the number of second and subsequent exchange transfusions. It is concluded that phototherapy is a valuable addition to the current methods of treatment of hemolytic disease due to rhesus incompatibility.