"Limited" phototherapy was used to treat full term babies with and without ABO-isoimmunization. Serum indirect bilirubin levels calling for irradiation during the first five days of life are summarized in a diagram (Fig. 1). Seperate indications were given for babies with different etiologies of jaundice, maturity (gestational age and birthweight) and indicational levels were correlated with the postnatal age (days and hours) of the infant. After a fall or harmless stagnation of bilirubin levels irradiation was stopped. Clinical assessment of the treatment of 17 babies with ABO-isiommunization, and 16 newborns without it and with hyperbilirubinemia is given in detail. Results were compaired with the outcome of treatment of 44 newborns who received the same care in every respect, but phototherapy. These control cases, selected by pairs, were similar. Exchange transfusion, based on identical indications was necessary in 13 of the 44 babies treated without and only 3 of the 33 infants treated with phototherapy. The billirubin levels of the treated babies were lower during the whole course of the irradiation. The difference in favor of the treated babies was significant on the second and third days of life incases of ABO-hemolytic disease, and from the 4th day of life in cases of hyperbilirubinemia. The values of standard deviations, important in view of the indication for exchange transfusion were also lower in the light treated babies. Successful irradiation required on average of 44 hours (30-72 hours) in cases of ABO-isoimmunization and 40 hours (18-50) in cases of hyperbilirubinemia, respectively. Re-elavation of the bilirubin level after finishing phototherapy was not observed.