The cytotoxicity of cadmium toward cultured bluegill fry (BF-2) cells was determined using several assay endpoints. The concentrations of cadmium causing a 50% decrease in colony formation, cell replication, uptake of neutral red, population growth (as determined by protein analysis), and uptake of [3H]uridine and 50% detachment of cells (as determined by protein analysis) were 0.03, 0.04, 0.08, 0.09, 0.12, and 0.21 mM cadmium, respectively. The neutral red assay was used to compare the relative sensitivities of bluegill BF-2 cells and RTG-2 cells, derived from the rainbow trout, toward four metals. The concentrations of cadmium, zinc, copper, and nickel causing a 50% reduction in the uptake of neutral red were 0.08, 0.19, 0.55, and 2.0 mM, respectively, with the BF-2 cells and 0.18, 0.64, 1.45, and greater than 10.0 mM, respectively, with the RTG-2 cells. The RTG-2 cells were less sensitive to the metals, in particular to nickel. The less stringent temperature requirements for growth, their greater sensitivity to pollutants, and their markedly shorter doubling time in vitro make the BF-2 cells the preferable cell line for ecotoxicity screening of aquatic pollutants.