A cold-adapted influenza A virus, CR-37 (H1N1), derived from genetic reassortment between A/Ann Arbor/6/60 (H2N2) cold-adapted variant virus and A/California/10/78 (H1N1) wild-type virus, was tested in Japanese adult volunteers. The CR-37 live virus preparation induced only low-grade clinical reactionsin volunteers for the first 3–4 days after inoculation. Two vaccinees who did not show any antibody changes became febrile (over 38–0 °C). Skin tests using the vaccine preparation and uninfected allantoic fluid were performed, and indicated that one of these two vaccinees was positive for the CR-37 vaccine preparation. A high proportion of the vaccinees whose sera had a haemagglutination-inhibition (HI) antibody titre against thevaccine strain of ≤ 64 before inoculation, seroconverted in both HI and neuraminidase-inhibition (NA1) antibody titrations, and only a few seroconverted in the titration of antibody against type-specific internal antigens. The serological examinations against heterotypic H1N1 variants indicated that the cold-adapted live influenza virus vaccine could induce a broad spectrum of HI antibody reactivity and immunity of long duration.