Seroconversion rates to measles, mumps and rubella (MMR) in children given MMR vaccine at 9, 12 and 15 months of age were assessed so as to recommend the optimum age for vaccination. A total of 164 infants were recruited, of whom 123 completed the study. Sera were tested pre-immunization and 4 wk after MMR vaccine, for the presence and titres of antibodies by the haemagglutination inhibition (HI) test and by enzyme-linked immunosorbant assay (ELISA). The pre-immunization results showed that levels of maternal antibody detectable by HI had disappeared by 9 months in all infants in the case of measles, but not in the case of mumps or rubella. Evidence for subclinical infection with the three viruses was found in 19 to 31 per cent of infants by 15 months of age. The responses to measles antigen by both HI test and ELISA were better (> 95%) at 12 or 15 months than at 9 months (80%). Vaccine failure was low at 12 or 15 months. The response to mumps antigen by HI antigen was also higher (92%) at 12 months than at 9 months (75%). Vaccine failure was less frequent at 12 months than at 9 months. The ELISA was found to be unreliable for mumps virus antibody testing. Rubella vaccine evoked good seroresponse (> 92%) at 9, 12 and 15 months, both by HI test and ELISA. Thus a better response to the MMR vaccine was obtained at or after 12 months of age than earlier. Hence, a dose of MMR may be given optimally at 12 months for children not previously immunized with measles vaccine. For those already given measles vaccine, the MMR may be given at 12 or 15 months.