A case of infectious leprosy in residential accommodation in the UK prompted a study of the cellular and humoral response to Mycobacterium leprae in two groups of individuals who were in contact with the index case for almost a year. In the younger staff group (mean age 44 years) 23 of 30 individuals had positive Mitsuda skin tests, 25 showed lymphocyte transformation to a soluble sonciate of M leprae and 2 had slightly raised IgM antibody concentrations to the terminal disaccharide of M leprae phenolic glycolipid-1. In the older group of residents (mean age 83 years) 7 of 36 individuals were skin-test positive, 25 of 33 were positive by lymphocyte transformation, but none had raised antibody levels. When retested on two further occasions, the same 2 individuals in the younger group still had raised antibody concentrations, 1 of whom had a persistent lepromin skin-test response for over 8 months and showed a pronounced increase in lymphocyte transformation to mycobacterial antigens. The findings suggest that transmission of M leprae may have occurred in these 2 contacts, who were therefore given 6 months' chemoprophylaxis with rifampicin.