Molecular variants of human T-lymphotropic virus type I (HTLV-I), which diverge significantly from the so-called cosmopolitan prototypes, have been discovered in Melanesia. In this study, HTLV-I IgG (I-IgG) prepared from seropositive healthy Japanese carriers was evaluated for its protective effect against a Melanesian isolate, HTLV-IMEL5, in rabbits. Normal IgG (N-IgG) prepared from seronegative healthy Japanese was used as control. Both preparations contained 50 mg/mL of IgG and I- IgG had a high neutralizing antibody titer, as determined by vesicular stomatitis virus--HTLV-I pseudotype assay. Of four experimental groups (A, B, C, and D), each with three rabbits, groups A and B were infused with 10 mL of N-IgG and I-IgG, respectively, and animals were challenged immediately by transfusion of 5 mL of blood from a rabbit infected with HTLV-IMEL5. Animals in groups C and D were immunized with 10 mL of I-IgG 24 and 48 hours, respectively, after being transfused with 5 mL of blood from the virus-infected rabbit. HTLV-I infection, as determined by seroconversion and verified by polymerase chain reaction, occurred in all rabbits in groups A and D after 2 to 6 weeks, but in none of the animals in groups B and C. These data indicate that I-IgG is protective against HTLV-IMEL5 infection when administered before or within 24 hours of transfusion with virus-contaminated blood. Moreover, our study shows that the neutralizing domains of the so-called cosmopolitan and Melanesian strains of HTLV-I are functionally indistinguishable.