In the United States, all blood used for transfusion is tested for the presence of antibodies to the structural components of the human T-cell lymphotropic viruses types 1 and 2 (HTLV-1 and -2). Based on such serologic tests, the prevalence of HTLV-1 infection is estimated to range from 0.016 to 0.1%. As a consequence of studies of patients with mycosis fungoides and some of their healthy relatives who are antibody negative but were found to carry the tax sequence of HTLV-1 in their lymphocytes and who had antibodies to the p40(tax) protein, a study was undertaken to determine the prevalence of the "tax-only" state in 250 healthy blood donors and other volunteers. Using PCR and Southern analysis for cell lysates and using Western blotting for plasmas, 8.6% of the blood donors proved to be tax sequence positive and antibody positive. Sequence analysis of specimens from 22 individuals proved that 20 of the sequences were homologous with that of HTLV-1 while 2 resembled the HTLV-2 sequence. The latter were obtained from volunteers of Indian origin. The possible clinical significance of the tax-only carrier state is discussed.