5,287 serum samples from 2 different sources in Finland, people possibly at risk and healthy blood donors, were tested for the presence of HTLV-I antibodies. No positive cases were found. The result suggests that this virus is not endemic in Finland. 10 cases gave repeatedly a low positive value in the enzyme immune assay (EIA) test but were confirmed negative with other tests that included western blot, passive agglutination and immunofluorescence. Four of these samples originated from healthy blood donors, 6 from other categories. Several of them showed restricted reactivity in western blots. Five HIV-positive sera, discovered during the study from people with possible risk factors, were also tested for HTLV-I but showed no reactivity.