• Trivalent measles-mumps-rubella virus vaccines, containing either Schwarz (two dosage levels), Urabe Am 9, and RA 27/3 strains or Moraten, Jeryl Lynn, and RA 27/3 strains, were evaluated in 174 children aged 14 to 24 months. Each of the three vaccines induced a nearly 100% seroconversion for measles and mumps and a full 100% seroconversion for rubella. Close home monitoring disclosed a high incidence and a relatively constant pattern of vaccine-associated reactions, similar for all three vaccines. Febrile reactions of greater than or equal to 39 °C between days 5 and 12 after vaccination were more frequent among the recipients of "high"-dose Schwarz strain vaccine (35%) than among the recipients of corresponding "low"-dose vaccine (23%) or the Moraten strain vaccine (13%). The Schwarz-Urabe Am 9-RA 27/3 was equally immunogenic and, with a low dose of measles virus, no more reactogenic than the Moraten-Jeryl Lynn RA 27/3 trivalent vaccine.