The human T-lymphotropic virus type I (HTLV-I) antibody profile of 216 asymptomatic carriers in Miyazaki, Japan, was analyzed in conjunction with the HTLV-I proviral DNA levels in their peripheral blood mononuclear cells (PBMC) determined by the semiquantitative polymerase chain reaction (PCR) method. The geometric mean HTLV-I titer by particle agglutination assay (PA) of 58 subjects (27%) with a high DNA level was 1:1, 240; 94 (44%) with a medium DNA level, 1:740; 38 (18%) with a low level, 1:476; and, 26 (12%) with an undetectable level, 1:263. Moreover, when the subjects were divided into four groups according to titer from high to low, the correlation between DNA level and antibody titer level was highly significant (p < 0.0001). HTLV-I antibody subclass by Western blot (WB) was determined for 78 randomly selected samples from these carriers. Immunoglobulin (Ig) M antibody was detected in 35 (45%). The mean PA antibody titer was higher in carriers with IgM antibody than in those without, at each detectable proviral DNA level. These findings suggest that HTLV-I antibody titer is related to proviral DNA level and also to the presence of IgM antibodies among those with proviral DNA of the same level. Seven carriers (9%) were negative for IgG antibody by WB, among whom the proviral DNA level was low or undetectable and the PA titer was also low. Three of these were positive only for IgM antibody.