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Powdered tobacco (Khaini tobacco) with the addition of lime is commonly used by the residents of Bihar, India. The tobacco/lime mixture is usually placed on the inner side of the lower lip within the gingivolabial groove. About 42% of the users keep it at the front, the rest move the tobacco towards the left or right side within the oral cavity. Carcinomas (so-called "Khaini cancers") develop mainly at the site where the tobacco is in close contact with the mucosa. Scrapings of the mucosa were taken at sites where the tobacco is kept, then smears were prepared, stained with the Feulgen reaction and fast green, and screened for micronuclei which indicate the occurrence of chromosome aberrations in the dividing cell population of the basal layer. An elevated frequency of cells with micronuclei was found in the oral mucosa of all 27 examined Khaini tobacco users (Munda and Santal tribes) compared to that of non-chewers of similar ethnic background and dietary habits. The induction of micronucleated mucosa cells seems to be due to genotoxic agents released from the tobacco/lime mixture. In vitro, an aqueous extract of the Khaini tobacco elicits chromosome aberrations and micronuclei in cultured human fibroblasts and Chinese hamster ovary (CHO) cells. No chromosome-damaging effect was observed following the application of lime or calcium hydroxide (Ca(OH)2). The micronucleus test on exfoliated cells can provide evidence of carcinogen exposure in the tissue from which cancers will develop. This approach combines all the advantages of in vitro short-term tests for genotoxic and carcinogenic agents with those of using an intact organism with all its defence mechanisms.