IgG and IgM class of antibodies to measles virus were estimated in the plasma samples of 211 unvaccinated infants living under overcrowded conditions in Pune, Maharashtra. IgG antibodies to measles virus, in high titres, were detected in 52.1 per cent and IgM antibodies in 31.3 per cent of the infants studied. Most of the infants with IgM antibodies to measles virus had not suffered from an attack of measles. A history of contact with an older child with overt measles was available for 13.3 per cent of the 211 infants studied. Among these infants with such a contact history, 82.1 per cent had IgM antibodies to measles virus. These observations suggest that many infants studied by us, had experienced subclinical measles infections and many times older children with measles at home or in the neighbourhood might have transmitted such infections.