The safety, infectivity, immunogenicity, and phenotypic stability of the cold-passaged (cp) candidate vaccine cp-45, a cold-adapted (ca), temperature-sensitive (ts) mutant of the J8 strain of human parainfluenza virus type 3 (HPIV-3), was evaluated in 114 children 6 months to 10 years old in a randomized, placebo-controlled, double-blind trial. The cp-45 vaccine was well tolerated when given intranasally to parainfluenza virus type 3 (PIV-3)-seropositive and -seronegative children. With 10⁴ or 10⁵ TCID₅₀ of cp-45 vaccine, 86% of seronegative vaccinees were infected, 83% of whom shed virus at a mean peak titer of 10^2.2 pfu/mL. Virus present in respiratory specimens retained the ts phenotype, and each of 86 PIV-3 isolates tested retained both the ca and ts phenotypes. One dose of 10⁵ TCID₅₀ of vaccine induced a serum hemagglutination-inhibiting antibody response in 81% of vaccinees; the geometric mean titer was 1:32. These studies indicate that the cp-45 HPIV-3 vaccine is satisfactorily attenuated, infectious, immunogenic, and phenotypically stable and merits further evaluation in infants and young children.