Primary cultures of chick embryo hepatocytes were maintained in Waymouth's MD 705/1 medium that was unexposed or had been exposed for 4 hr to 366 nm ultraviolet (UV) light. The cultures were assayed for 3,3',4,4'-tetrachlorobiphenyl (3,3',4,4'-TCB)-mediated porphyrin accumulation and ethoxyresorufin-O-deethylase (EROD) induction. A decrease in 3,3',4,4'-TCB-mediated porphyrin accumulation, due in part to tryptophan depletion after exposure of the cell culture medium to UV light, was observed. In contrast, EROD activity of cells maintained in irradiated medium was shown to increase over basal levels, possibly due to the presence of photoproducts of medium components. Further studies indicated that photoproducts obtained after irradiation of a concentrated aqueous solution of tryptophan were inducers of EROD activity in primary chick embryo hepatocytes.