• Solutions of bilirubin containing human serum albumin were exposed in vitro in the presence of 10 μmol/L of tin and zinc metalloporphyrins at 37°C for 30 minutes to light sources used clinically for phototherapy of neonates. Bilirubin in the model solutions was photodegraded to approximately 60% of dark control in cool white light (17 μW/cm2 per nanometer). The presence of zinc protoporphyrin and zinc mesoporphyrin further reduced the bilirubin concentration slightly, but the tin analogues caused a significant enhancement of degradation to 35% and 25% of dark control, respectively. The results provide evidence that the zinc and tin metalloporphyrins are photosensitizers capable of enhancing the native photodegradation of bilirubin in biologic matrices, but that the tin compounds are more potent. The metalloporphyrin time course, dose-response curve, oxygen effects, and efficacy of phototherapy light sources were also studied.