The effect of simultaneous administration of Schwarz-strain measles virus vaccine and Cendehill-strain rubella virus vaccine as a single bivalent injection was evaluated in 346 children susceptible to both diseases. With placebo and monovalent controls, four bivalent preparations were used in a double-blind protocol to determine the effects of variations in the concentration of one or both components. No unusual reactions occurred and simultaneous administration did not enhance clinical reactivity. For measles, the seroconversion rate was 97% for monovalent vaccine and 97% to 100% (average 98%) for bivalent vaccines. For rubella, the rate was 100% for monovalent vaccine and 94% to 100% (average 99%) for bivalent vaccines. Thus, the vaccines were as safe and effective when given together as when given separately.