Both population-based incidence rates and relative frequencies of oral and pharyngeal cancer seen in six major cancer hospitals in India indicate that these forms of cancer occur frequently. Case-control studies reveal that these cancers are associated with tobacco chewing and bidi smoking. Experimental studies on a variety of chewing tobacco used commonly in western India revealed that it contains N-nitrosonornicotine and 4-(methyl-nitrosamino)-1-(3-pyridyl)-1-butanone in microgram quantities per gram of tobacco. A crude alcoholic extract of tobacco containing these nitrosamines was mutagenic in histidine-deficient Salmonella typhimurium strain TA98 in the presence of a 9000 X g supernatant fraction. Gavage feeding of this extract for 7, 15 and 30 days induced activity of mixed-function oxygenases. Feeding of the tobacco extract by gavage or in diets induced lung and liver tumours in Swiss mice.