Betel quid ingredients--betel nut, betel leaf, lime, catechu and tobacco--were tested separately and in various combinations for carcinogenicity, using hamster cheek pouch as the experimental site. The four modes of administration used were (1) tri-weekly painting of the cheek pouch with aqueous extracts of test materials, (2) deposition of replaceable wax pellets containing the test material, (3) gelatin capsules containing the powdered material and (4) insertion of natural material into the pouch for trauma and direct exposure. Untreated controls and standard carcinogen DMBA-treated controls were also maintained. A total of 317 young adult golden Syrian hamsters (Mesocricetus auratus) used for the experiments were killed in two age groups: 6-12 months and 13-24 months, only when signs of general debility were observed. In the untreated controls, animals were free of any malignancy. In the experimental series, various betel quid ingredient combinations under test induced both oral and gastric lesions ranging from massive atypia and precancerous lesions to frank carcinomas. Maximum lesions were observed in the groups receiving betel nut, lime and tobacco combinations and in the polyphenol fraction of betel nut containing major tannins. The mode of administration of test material resulted in distinct differences; tri-weekly paintings giving oral lesions in the range of 22-23% and gastric lesions 39-48%; the same material given either through the replaceable gelatin capsule or in natural form induced 69% oral lesions and 63 to 82% gastric lesions. Overall evaluation of the data of all the four series confirms the potent carcinogenicity of betel nut, particularly its tannin-containing polyphenolic fraction and its combination with lime and tobacco. Maximum oral lesions induced in the hamsters by continuous exposure to capsules and natural material, highlight the direct relationship of frequency of chewing in habitual chewers with oral carcinogenesis. The high incidence of gastric (forestomach) lesions invites special attention.