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During a field study in South India in 1989, faecal specimens were collected from residents in villages and the town of Vellore in South India. Examination of the faecal specimens revealed that virtually the whole population carried commensal bacteria resistant to trimethoprim, ampicillin and chloramphenicol. Most specimens contained more than one type of bacterium resistant to each antibiotic. There was less resistance to nalidixic acid, with a higher proportion in the town (33%) than in the villages (13%). Although there was little cross-resistance of the ampicillin-resistant strains to later generation cephalosporins, 50% were resistant to the combination of amoxycillin and clavulanic acid. There was no significant cross-resistance of the nalidixic acid-resistant strains to fluorinated 4-quinolones, despite the free availability of ciprofloxacin and norfloxacin in the area. The probable reason for the high incidence of resistance to first generation antimicrobials is the extensive use of these agents, coupled with continuous exposure to large numbers of faecal micro-organisms.