The cytotoxic action of a series of mineral dusts has been studied using Chinese hamster V79-4 cells obtained from three separate laboratories in the UK. The dusts which have been studied include samples of asbestos from the Union Internationale Contre le Cancer reference series and an internationally available sample of quartz (DQ12). Besides comparing results obtained with the three different cells, two alternative methods of exposing the cells to the dusts were also compared. The toxicity assay employed uses the inhibition of colony formation from a single cell suspension as a measure of toxicity. The numbers of colonies which grow in the presence of the various concentrations of the dusts are expressed as percentages of the colonies which grow in the untreated control cultures. The absolute toxicity of a dust was found to depend principally on the method of exposing the cells to the dust. However, differences in the relative toxicities of the dusts depended on the source of the cells. The implications of these results for experiments in which Chinese hamster V79-4 cells are used in in vitro test systems to assess the potential pathogenicity of mineral dusts are discussed.