Neutralizing antibody following Jeryl Lynn strain live mumps virus vaccine persisted for at least three years following vaccination without substantial decline in titer. The pattern for mumps antibody persistence after vaccine paralleled that following natural mumps infection. No cases of mumps occurred among 18 vaccinated children in families who were exposed to infection by contact with natural mumps cases which occurred in siblings 19 to 32 months after the vaccine had been given. Similarly, no cases of mumps occurred among 32 children in classrooms who were exposed to natural mumps during the third year following vaccination. Children who received the mumps virus vaccine developed antibodies against soluble (S) antigen as well as viral (V) antigen indicating that infection-type immunity had been established. The available data indicate that the protection afforded by the mumps virus vaccine will be lasting.