A B S T R A C T:Seven natural clay minerals (two kaolinites, three montmorillonites, a hectorite and a palygorskite) were tested for their eytotoxicity in vitro using an established eytotoxieity assay. Cellular viability, levels of lactate dehydrogenase, N-acetyl-fl-o-glucosaminidase, lactic acid and total cellular protein were measured in cells of the macrophage-like cell line, P388D,, after 48 h incubation with each clay. There was a significant relationship between BET/N 2 surface area of the clays and cytotoxieity but in two cases the level of eytotoxicity was apparently influenced by the presence of quartz or cristobalite in the samples. Results are discussed in the light of the available animal and epidemiologieal data relating to clay minerals.