In a series of clinical studies of a combined measles (Schwarz strain), mumps (Jeryl Lynn strain), and rubella (Cendehill strain) vaccine, 1,481 children received the vaccine or a placebo. The vaccine did not cause any significant reactions. The frequencies of mild, transient fever or rash or both in triple-susceptible vaccinees were similar to those that follow use of Schwarz strain measles vaccine alone. Measles, mumps, and rubella seroconversion rates in triple-susceptible vaccinees ranged from 95% to 100%. Geometric mean antibody titers were as high as those that usually result from use of these same virus strains as monovalent vaccines.