The cytotoxicity and genotoxicity of refined borax and borax ores were studied in cultured mammalian cells. In V79 Chinese hamster cells, C3H/lOTl/2 mouse embryo fibroblasts, and diploid human foreskin fibroblasts, crude borax ore, kernite ore, and refined borax were all cytotoxic. The lowest concentrations at which cytotoxicity was observed were 0.02 mg/ml and 0.1 mg/ml for borax ore in C3H/lOT1/2 and human fibroblasts, respectively, 0.2 mg/ml for kernite ore in both cell types, and 0.1 mg/ml for refined borax in both C3H/IOT1/2 and human fibroblasts. The cytotoxicity was dose dependent above these concentrations. The concentrations of borax ore, kernite ore, and refined borax that reduced the relative plating efficiency to 50% were approximately 3.2, 1.6, and 0.8 mg/ml, respectively, in human fibroblasts and were 0.8 mglml for all three substances in C3H/lOT1/2 cells. All three borax samples were not significantly mutagenic in assays for mutation to ouabain resistance in human fibroblasts and C3H/ 10T1/2 cells and were at most only weakly mutagenic in an assay for mutation to 8- azaguanine resistance in V79 Chinese hamster cells. Refined borax did not induce neoplastic transformation in C3H/lOT1/2 cells. Crude borax ore and kernite ore induced weak transformation that was not dose-dependent and was not reproducible in another experiment. Therefore, borax and its ores are cytotoxic to mammalian cells at high (mg/ ml) concentrations and are at most weakly mutagenic but not significantly oncogenic as measured in a cell transformation assay.