Serum and peripheral blood cell samples from eleven relatives of a T-cell leukemia patient with human T-cell leukemia/lymphoma virus (HTLV)-associated disease, were investigated for the presence of HTLV antibody and antigen expression. In addition to the patient, three family members were seropositive and a wide range in HTLV antibody titer was observed. The father of the patient showed the highest titer (173,000) and carried HTLV p19+ cells in his peripheral blood. Upon induction of proliferation of these cells by short-term culture in the presence of phytohemagglutinin (PHA) and 12-O-tetradecanoyl phorbol-13-acetate (TPA), an increase from 1% to 28% HTLV p19+ cells was observed confirming previous findings that HTLV p19 expression was correlated with proliferative activity of the host cells. In none of the other seronegative or seropositive relatives of the patient HTLV p19 expression was revealed when tested in freshly isolated mononuclear cells, upon short-term incubation in PHA + TPA or after prolonged culture for 2 or 3 passages in medium containing T-cell growth factor. The results from HLA typing studies did not provide any evidence for HTLV related abnormalities in haplotype expression. Our data confirmed the earlier notion of a prevalence of HTLV infection within families of patients with HTLV-associated disease. It is furthermore obvious from our results that a normal individual may possess high titer HTLV antibody and circulating HTLV p19+ cells without showing signs of disease.