This report deals with certain aspects of phototherapy of infants with hyperbilirubinaemia. The first part of the report concerns light measurement at different distances from a phototherapy unit equipped with eight fluorescent tubes. At the actual distance of treatment, there was a considerable lack of homogeneity with the largest irradiance below the center of the unit. The second part of the report is a study of the formation of different isomers of bilirubin detected by use of HPLC. The research was mainly done in vitro by irradiation of bilirubin solutions. Three different fluorescent tubes were used which emitted white, blue and green light. The green light was most efficient in producing Z-lumirubin, which is considered as the main isomer responsible for excretion of bilirubin from infants. Green light may be safer to use than blue light and may in the future be the wavelength of choice in phototherapy. 43 refs., 50 figs.