SummaryPhototherapy results in the conversion of bilirubin to more water-soluble isomers. Six clinically used phototherapy lamps which differ in their emission spectra have been compared in their ability to produce configurational and structural isomers of bilirubin in vitro. For all of the lamps, the initial rate of configurational isomerization was highly correlated (r=0.969) with the intensity of irradiation falling within the bilirubin absorption band. The percentage of the total bilirubin converted to the configurational isomer at equilibrium was dependent upon the spectral distribution of the lamp, and was greatest (26.2 ± 1.3%) with the special blue lamp, which has a narrow spectral output centered at 445 nm. The rate of formation of the structural isomer, lumirubin, was generally dependent upon the intensity of irradiation within the bilirubin absorption band.