The efficacy of fluorescent green light phototherapy was compared with that of blue light phototherapy in the treatment of full-term infants with hemolytic disease and jaundice caused by ABO incompatibility. The efficacy of the treatment was expressed as actual (milligrams per hour) and quantum (milligrams per hour per square centimeter per megawatt) efficiency, taking into account the differential emission of energy from the green versus the blue fluorescent tubes. No statistically significant difference in the rate of serum bilirubin photodegradation was found between the two groups after treatment for 84.6 +/- 14.1 hours versus 81.5 +/- 14.2 hours with the green and the blue phototherapy, respectively. These results, coupled with the known effects of the blue light on the genetic apparatus of mammalian cells, support the application of the green light phototherapy for the treatment of neonatal hyperbilirubinemia caused by ABO incompatibility.