Rubella vaccine combined with measles and mumps vaccines was administered in a single injection to children of 1 to 5 years of age. All three vaccines were serologically effective, and the clinical reactions caused by measles vaccine were considerably alleviated, when 6 x 10(3) PFU of rubella and 10(4) TCD50 per dose of mumps vaccines were combined with 5 x 10(4) TCD50 of measles vaccine. When a larger amount of mumps vaccine (3 x 10(5) TCD50/dose) was used, it caused interference with the rubella and measles viruses, i.e., the antibody response to rubella virus became poor and the incidence of clinical reactions to measles virus decreased. On the other hand, when 5 x 10(5) TCD50/dose of measles vaccine was combined with 10(4) TCD50/dose of mumps vaccine, the clinical reactions to measles virus were decreased but were almost the same as those induced by this vaccine alone.