Inactivated viral vaccines for parainfluenza virus types 1, 2, and 3 were prepared in embryonated hens' eggs, type 1 in the allantoic cavity, types 2 and 3 in the amniotic sac. The vaccines were given to 36 young infants, 28 of whom lacked neutralizing antibody for the virus corresponding to the vaccine administered. Ten infants received parainfluenza 1, ten received parainfluenza 2, and 16 received parainfluenza 3 vaccine. All infants received two injections at an interval of one month, and each infant given the parainfluenza 1 and 3 vaccines received an additional injection 41 to 163 days after the second injection. These vaccines were 90% to 100% effective in inducing a neutralizing antibody response. The infants given parainfluenza 2 vaccine responded following two injections, whereas a majority of those given parainfluenza 1 and 3 vaccines required an additional injection at a later time. Repeated freezing and thawing appeared to inactivate the serum neutralizing activity induced by type 2 vaccine.