The absorption spectrum of serum samples from infants with bronze baby syndrome (BBS) showed spectral features typical of bilirubin (i.e., Λ max at approximately 460 nm), as well as an intense band peaking around 400 nm and a generalized broad absorbance in the near-UV and in the 600–700 nm spectral region. The two latter features were not observed in the absorption spectra of control sera; moreover, the BBS sera exhibited multiband emission spectra with maxima at 585, 619, and 670 nm. This finding indicates that the 400-nm absorption band does not reflect the presence of residual hemoglobin in the sera because hemoproteins are known to be nonfluorescent in the red region. Instead the observed emission spectra are characteristic of porphyrin compounds. The porphyrins extracted from BBS sera were analyzed chromatographically. The results of paper chromatography led to the conclusion that the two porphyrins present in the ether fraction are Cu2+-proto and Cu2+-coproporphyrin, whereas the porphyrin in the aqueous phase is Cu2+-uroporphyrin.Visible light-irradiation of diluted sera from BBS newborn infants caused an increase of the brown color of the solution corresponding with a disappearance of the typical porphyrin fluorescence and an increase of the absorbance in the near-UV and red spectral regions. Extraction of porphyrins from 15-min irradiated BBS sera showed that an essentially total destruction had occurred.In order to verify the responsibility of Cu2+-porphyrins in inducing the BBS discoloration, we irradiated 8-ml samples of serum obtained from normal cord blood in presence of Cu2+-proto, Cu2+-copro, and Cu2+-uroporphyrin, which were added at a concentration of 3.28 μg/ml. Cu2+-porphyrins were slowly destroyed and the consequent spectral changes closely resembled those observed from BBS sera from phototreated patients. The addition of equimolar bilirubin to the aqueous solution of Cu2+-porphyrins bound to albumin enhanced the first order rate constant for porphyrin photodestruction from 0.12 sec-1to 0.49 sec-1.