Two mumps-measles vaccine combinations were evaluated for their reactogenicity and immunogenicity in children aged 14 to 20 months. The Urabe Am 9-Schwarz combination vaccine was given to 108 double seronegative children. The seroconversion rate at six weeks after vaccination was 99.1% for measles (haemagglutination-inhibition test) and 92.6% for mumps (neutralization and haemolysis-in-gel tests). The Jeryl Lynn-Moraten vaccine was administered to 85 double seronegative children; the seroconversion rates were 95.3% for measles and 83.5% for mumps. The reported post-vaccination signs and symptoms resembled those seen after monovalent measles vaccine but were more accentuated. Fever over 37.5 C degrees was reported in 66.7% and unusual restlessness and irritability in 68.5% of the Urabe Am 9-Schwarz double seronegative vaccines compared to 55.3% (p less than 0.05) and 54.1% (p less than 0.05), respectively, in the recipients of the Jeryl Lynn-Moraten vaccine. These relatively high reaction rates probably reflect the close observation of the children by their parents during the study. Nevertheless, the tendency towards increased reaction rate and, possibly, reduced immunogenicity of bivalent mumps-measles vaccines as compared to the corresponding single vaccines should be taken into account in the planning of large scale vaccination of young children.