Beginning in 1979, OKA and KMcC strains of varicella zoster virus (VZV) vaccine were administered to 369 healthy seronegative children in a sequence of ten comparative clinical trials. Postimmunization clinical reactivity was minimal with the OKA vaccines but was unacceptably high (32%) with the KMcC passage-40 vaccine. Ninety-three percent to 100% immunogenicity was noted by fluorescent antibody assay and in vitro lymphocyte proliferation to VZV antigens. Follow-up studies demonstrated persistence of antibody and in vitro lymphocyte proliferation responses and protection or modification of infection nine to 48 months after immunization. Only five episodes of mild varicella occurred in children in whom seroconversion had occurred. These episodes were noted after at least 281 known varicella exposures. Vaccine virus reactivation as zoster had not occurred in any child.