Photodegradation of bilirubin in vitro has been investigated by using monochromatic light supplied by an argon ion laser selecting the 457.9, 488.0 and 514.5 nm wavelengths. Bilirubin was examined in chloroform, in aqueous solutions and in human serum under different experimental conditions of concentration, laser power and time of irradiance. Photodecomposition was followed by optical density measurements on the absorption maximum of the electronic band at 460 nm. The rate of degradation of bilirubin was found to be only slightly affected by the wavelength of the exciting lines provided they fall within the absorption band. In particular it was shown that any wavelength, lambda, is equally effective if the corresponding absorbance, A lambda, exceeds a minimum value of 5-10%. In the aqueous solutions, light with lambda greater than 470 nm has been found to be largely effective in the photodegradation of bilirubin in vitro.