It is known that the human T lymphotropic retrovirus type I (HTLV-I) preferentially selects lymphocytes expressing the CD4 phenotype in vivo and in vitro. The present study shows that the emergence of double-positive (DP) CD4/CD8 cells was a constant, even if transient, phenomenon occurring in early phases after the in vitro HTLV-I infection of adult human peripheral blood mononuclear cells (PBMC). Moreover, purified CD8+ lymphocytes, isolated from human PBMC after challenge with HTLV-I, gave origin to a relatively stable DP CD4/CD8 cell line after a few weeks in culture. Conversely, isolated CD4+ T lymphocytes did not show DP emergence during either the early phases of HTLV-I infection or long-term culture. One of the DP cell lines was maintained in culture for more than 1 year and was characterized on the basis of virological and phenotypic features. This cell line bore HTLV-I sequences as demonstrated by PCR analysis, and 60-90% of the DP cells expressed the virus core protein p19. In addition the phenotype of this DP cell line infected with HTLV-I highly expressed antigens associated to activation such as CD45R0, CD18, and CD54.