Quadruple skin testing with new tuberculins was used to evaluate the effects of previously administered BCG Madras in children attending schools in the slums, or living in Kopri Leprosy Colony in Bombay. There were differences between schools both in the level of sensitisation of children without BCG scars and in the effects of BCG vaccination. Results obtained at one school resembled those obtained in a previous study in Agra, where BCG was thought to be ineffective. Results from the other schools and Kopri were more like those previously reported from Ahmednagar, where BCG was considered to be much more effective. Thus within the same city groups of children of the same social status may vary widely both in their contact with mycobacteria and in their capacity to benefit from BCG vaccination.