Phototherapy results in transformation of bilirubin to more water-soluble isomers. The efficacy of monochromatic visible light from 350 to 550 nm in the fastest photoisomerization reaction was quantitated by high-pressure liquid chromatography. The most effective wavelengths in vitro (i.e., leading to greater than 25% photoisomer) were in the blue spectrum from approximately 390 to 470 nm. Green light (530 nm) was not only ineffective for production of photoisomer, but capable of reversing the reaction. The results indicate that any clinically useful phototherapy unit must include the blue portion of the visible spectrum, and suggest that the effectiveness of phototherapy may be increased by elimination of green light.