Methodology was developed for quantifying the photocytotoxicity of fluoranthene to a gill cell line from rainbow trout for future use in screening polycyclic aromatic hydrocarbons for their relative photocytotoxicity to fish. Solubilization in a modified culture medium was achieved with and without foetal bovine serum (FBS) and with and without dimethyl sulfoxide (DMSO). FBS caused most of the fluoranthene to remain in solution and blocked photocytotoxicity if present during UV irradiation. DMSO had little effect on fluoranthene distribution in cell cultures but caused cells to be slightly more sensitive to the phototoxicity of fluoranthene. The indicator dyes alamar BlueTM and 5-carboxyfluorescein diacetate acetoxymethyl ester were used to quantify cytotoxicity in two different ways-singly in two separate assays, and mixed together in a novel single assay, which saved time and material. With UV irradiation for 2 hr at a photon fluence rate of either 1.4 mu mol UV-B/m(2)/sec (UV-A:UV-B, 1.5) or 1.1 mu mol UV-B/m(2)/sec (UV-A:UV-B, 9.7), both dyes indicated increasing loss of viability with increasing doses of fluoranthene. EC, values ranged from 18 to 44 ng/ml (89-217 nM), with the alamar Blue assay being slightly more sensitive. (C) 1997 Elsevier Science Ltd.