As part of a series of investigations to determine the effect of sensitisation by environmental mycobacteria on the efficacy of BCG vaccination in India, this study was carried out in Ahmednagar in Maharashtra. A preliminary skin test survey showed that the rate of sensitisation with age was much lower than in Agra, the site of a previous study, and BCG vaccination scars were associated with considerable enhancement in sensitisation to Tuberculin and other reagents. It was possible to set up prospective BCG vaccination studies in pre-school and primary and secondary school children. Follow up with skin tests were carried out 1 and 2 years later. By the second year, results were obtained almost identical with those 10 years after BCG administration in the UK. On this basis it is proposed that the vaccine is likely to provide a considerable level of protection in Ahmednagar. The results of this study also resemble those obtained in the very youngest age group studied in Agra. The marked differences between Indian towns strongly suggest the influence of exposure to mycobacteria in the environment.