Riboflavin in whole blood, G-6-PD activity in erythrocytes and amino acids in plasma of infants irradiated by broad and narrow spectrum blue fluorescent light were determined. Riboflavin was reduced by both types of light. Loss of G-6-PD activity during phototherapy with subsequent hemolysis was observed in some infants under each type of light source, both in vivo and in vitro. This phenomenon may be related to the indirect effect of riboflavin reduction and not to a direct effect upon the enzyme. The reduction of the plasma concentration of some amino acids observed under broad spectrum irradiation but not under pure blue light may not be of significance, but is of concern. These data indicate that all fluorescent light sources in current use for phototherapy produce biologic effects of importance beside the primary purpose of the treatment to reduce hyperbilirubinemia. It is apparent from the results of these studies that phototherapy light penetrates more deeply than the superficial skin to exert photochemical effects.