Methodology was developed for presenting to fish cells in culture whole-water samples without extraction and used to evaluate the toxicity to a rainbow trout gill cell line, RTgill-W1, of more than 30 whole-water samples collected from a paper mill over approximately a year of operation. Presentation to cells was achieved by adding to water samples the amounts of salts, galactose and sodium pyruvate, as solids, that were necessary to give concentrations and osmolality of the basal growth medium, Leibovitz's L-15. Cell viability was measured with three fluorescent indicator dyes: alamar Blue™ for metabolism, 5-carboxyfluorescein diacetate acetoxymethyl ester (CFDA-AM) for membrane function, and neutral red for lysosomal activity. Eighteen samples were tested with the Daphnia lethality bioassay and 11 of these were toxic. None of these were judged cytotoxic to RTgill-W1. Sixteen samples were tested with the rainbow trout lethality bioassay and only one was toxic. This sample was also the only sample that was cytotoxic to RTgill-W1. Therefore, these methods for presenting water samples and measuring their cytotoxicity to RTgill-W1 are a promising substitute for toxicity tests of industrial effluent with rainbow trout but not with Daphnia. © 2002 Elsevier Science Ltd. All rights reserved.