There have been several recent reports on the high prevalence of serum antibodies to human T lymphotropic virus-1 (HTLV-1) in isolated populations residing in the coastal areas and highlands of Papua New Guinea. In the absence of significant cases of clinical disease, it has been surmised that this reactivity might be the consequence of serologic recognition of yet undefined human retroviruses or parasite antigens. These observations prompted an investigation of the prevalence of anti-HTLV-1 antibodies among members of the Ngalum tribe that dwells in a secluded highland valley in the eastern Jayawijaya Mountains of Irian Jaya, Indonesian New Guinea. Of 165 tribespeople, 85 (52%) were positive for IgG antibodies to HTLV-1 in an indirect enzyme-linked immunosorbent assay. Eighty-two were more than 10 years of age. On the Western blot, all positive sera reacted strongly with the p19 core antigen, but recognition of the envelope antigens, gp46 and gp21, was conspicuously absent. Thirty-four of the 85 villagers with these indeterminant blots had active Plasmodium falciparum infections, but antibody absorption studies with HTLV-1 and P. falciparum erythrocytic stage antigens failed to confirm suspected serologic cross-reactivities. Thirty-three others had acute malaria and/or high titers of anti-malaria antibodies but were seronegative for HTLV-1. We suspect that indeterminant Western blots for HTLV-1 reflect antibody responses to related latent retroviruses that are activated as a consequence of immunosuppression following malaria infection and chloroquine therapy.