Changes are reported in the DNA, RNA, protein and brush border enzymes in the small intestine mucosal lining cells and gut lumen, induced by maintaining rats on a diet containing chrysotile asbestos for 10 months. Intracellular levels of RNA, DNA and protein remained unchanged but significant alterations, consistent with a mineral-induced cytotoxicity, were found in the lumen level of DNA (increased) and RNA (decreased) in asbestos-treated rats. Most intracellular enzyme levels were consistently, but not significantly, elevated in animals maintained on diets containing asbestos whilst the activities within the lumen were significantly higher than those found in normal animals. The presence of cigarette smoke alone in the diet induced changes in intracellular RNA and lumen ATPase but the combined effect of cigarette smoke and asbestos was rarely different from the alterations induced by asbestos alone on the majority of parameters studied.