We examined the sera of family members of human T cell leukemia virus type I (HTLV-I) seropositive pregnant women who had visited Kagoshima City Hospital since 1986, and studied the routes of transmission of HTLV-I. A new enzyme linked immunosorbent assay (ELISA) for detecting the antibody to an HTLV-I tax gene product, p40tax, has recently been developed. By this ELISA method, the positive rate of anti-p40tax among HTLV-I seropositive subjects, including 96 pregnant women (index subjects), 26 mothers, 13 husbands, and 13 children was investigated. The percentage positive for anti-p40tax among pregnant women, mothers, husbands, and children was 41.6, 50, 53.8 and 53.8%, respectively. This means that the positive rate of anti-p40tax remains almost constant with increasing age. The rate of mother-to-child transmission of HTLV-I was significantly higher in p40tax seropositive (29.6%) than in seronegative mothers (8.1%). The positive rate of anti-p40tax in transmission from husband to wife (29%) and through blood transfusion (17%) was lower than the overall prevalence (46%). Thus, these data suggest that p40tax antibodies are associated with the frequency of HTLV-I transmission and with the differences in the transmission routes.