In developing countries, antibiotics are the most common drugs sold, and some data suggest that they are frequently misused. In order to describe the pattern of antibiotic use in a periurban community in Mexico City, 1659 randomly-selected households were visited and an interview with the housewife was carried out. Six local drugstores also were selected at random. A social worker made six visits to each pharmacy, observed the events during the purchase of the drug and applied a structured questionnaire to the customer immediately after the transaction. Of 8279 individuals, 425 (5%) said that they had used at least one antimicrobial in the preceding 2 weeks and antibiotics were the majority (29%) of the drug sales. The main perceived reasons for drug use were acute respiratory tract ailments and gastroenteritis. Interviewees reported that antibiotic therapy was given in 27% of respiratory diseases and in 37% of all diarrheal episodes. The drugs most commonly reported were: penicillins, erythromycin, metronidazole, neomycin, cotrimoxazole and tetracyclines. While self-medication and drug purchases without medical prescription were common, the majority of antibiotics were prescribed by a physician. Approximately two thirds of individuals using an antibiotic said they had used it for less than 5 days and 72% of the purchases were for insufficient quantities of drugs. Our data suggest that antibiotics are frequently misused and they support the need to assess the determinants of self-medication, health-seeking behavior and physician prescribing practices. The need for effective educational programs to improve prescribers' decisions is stressed.