Clinical and laboratory examinations in two permanently observed children's institutions were carried out in 1968--1978. Altogether, 241 children were examined virologically 759 times, of them 181 children were found to be truly healthy. In the epidemic period the latter yielded virus in 3.7%, in the interepidemic period in 1%. Unlike asymptomatic infection, transitory virus carrier state was not accompanied by antibody production, and in a number of such cases subsequently led to an overt clinical disease.