Two alloreactive T cell clones with anti-HLA-DR1 specificity showed significant alterations of cognitive and functional characteristics after infection with human T cell leukemia virus (HTLV-I). Similar to HTLV-I-infected lines derived from immunologically uncommitted lymphocytes, the transformed clones displayed blastogenic mixed lymphocyte culture (MLC) responses to cells carrying any of the allelic variants of human HLA-D/DR antigens, including self. Although these two clones were originally able to provide allospecific help only to B cells expressing the DR1 antigen, after infection with HTLV-I they stimulated B cells of any HLA-D/DR phenotype to produce immunoglobulin in cultures. The helper inducer activity of the transformed clones remained susceptible to the effect of monoclonal antibody anti-LDA1 that inhibits the helper function of normal human T cells. One of these clones (207TK), which before infection specifically killed DR1-positive target cells, lost its killing ability. The other clone (19TK) although originally noncytotoxic, acquired natural killer-like function after transformation. Study of the rearrangement of the genes coding for the beta-chain of the T cell antigen receptor revealed no differences between the wild (noninfected) and mutant (infected) clones. There was, however, an increased level of this message, as well as of the message encoded by the beta-chain gene of HLA-DR in the mutant clones. Such changes may be related to transacting transcriptional effects induced by the human T cell lymphotropic virus HTLV-I.