To examine the value of vaccinating children against measles at age 4-5 months three groups of children in Togo were randomly assigned to receive high-dose Edmonston-Zagreb, high-dose Schwarz, or AIK-C vaccines at that age. Two other groups were vaccinated at age 8-10 months, with either the standard Schwarz vaccine or the same batch of AIK-C as for early immunization. For the 55% of children who were seronegative before vaccination at age 4-5 months, the seroconversion rates were 96% with the AIK-C, 94% with the Edmonston-Zagreb, and 50% with the high-dose Schwarz. The immune response rates for children who were seropositive before vaccination were estimated to be around 50% with the high-dose Edmonston-Zagreb and AIK-C strains and 10% with the Schwarz strain. The seroconversion rates for vaccination at 8-10 months were 87% for the AIK-C and 68% for the Schwarz strains. Thus vaccinations with the high-dose Edmonston-Zagreb or the AIK-C strain at 4-5 months as good as vaccination with the AIK-C strain at 8-10 months better than vaccination with the standard dose Schwarz strain at 8-10 months. These findings require confirmation by further investigations, particularly epidemiological studies, but they already provide evidence that administration of AIK-C and high-dose Edmonston-Zagreb strains at 4-5 months is at least as effective as vaccination with the standard strains at 8-10 months.