Under in vitro conditions, theophylline accelerates the rate of bilirubin photodestuction sensitized by riboflavin, but does not do so in absence of this dye. The effect depends on the concentrations of theophylline and/or riboflavin, on the bilirubin/albumin ratio, and seems to implicate bilirubin unbound to serum albumin. Possible causes of the theophylline action and clinical implication regarding thephototherapy of neonatal jaundice are discussed.