To examine the role of the Plasmodium falciparum Exp-1 blood-stage protein in producing antibodies that cross-react with human T-cell lymphotropic virus type I (HTLV-I) proteins, we studied sera from Indonesian volunteers who seroconverted to malaria after transmigrating to an area where malaria is hyperendemic. Samples from Philippine volunteers, that were used in a prior study that examined malaria antibodies that cross-react with HTLV-I proteins, were also used. Eighty-three percent of the Indonesian transmigrants developed antibodies against the malaria Exp-1 protein by 6 months postmigration. Of these malaria seroconverters, 27% developed false-positive HTLV-I enzyme immunoassay (EIA) immunoreactivity, as indicated by indeterminate HTLV-I Western blot banding patterns. Five of the six Philippine samples tested were HTLV-I EIA false positive and Western blot indeterminate. When a recombinant Exp-1 protein was used in blocking experiments, the HTLV-I Western blot immunoreactivity of sera from both groups was either completely eliminated or greatly reduced. No effect on the Western blot immunoreactivity of truly HTLV-I-positive sera was seen. To determine if immunization with the recombinant Exp-1 protein could elicit the production of HTLV-I antibodies, six mice were inoculated with the recombinant protein. Following administration of three 50-microgram doses of the protein, four of the six mice developed antibodies that cross-reacted with HTLV-I proteins on Western blot. These results indicate that the immune response against the malaria Exp-1 protein may result in HTLV-I-cross-reacting antibodies that can lead to false-positive EIA and indeterminant Western blotting results.