CD4-CD8- double-negative (DN) and CD4+CD8- T cell clones were derived from splenic precursors resistant to killing by anti-Thy-1, -CD5, -CD4 and -CD8 monoclonal antibodies and complement. Both DN and CD4+ clones express functional T cell receptor (TcR) alpha/beta and exhibit strong autoreactivity in vitro. DN cells can be induced to proliferate by dendritic cells (DC) of all haplotypes tested, although this activation is inhibited by antibodies specific for I-A determinants expressed on the stimulatory DC. In contrast, CD4+ clones only respond to syngeneic or I-Ad-compatible DC. Both DN and CD4+ autoreactive clones do not proliferate when cultured with class II+ H-2d normal or tumor macrophages and B cell lines or with class II-transfected L cells, suggesting that these cells recognize self peptides only present on the surface of DC. Despite their phenotype resembling that of immature thymocytes and their inability to interact directly with B lymphocytes, DN cloned T cells, like CD4+ T cells, exhibit nonspecific helper functions and can induce polyclonal B cell proliferation and differentiation. DN TcR alpha/beta+ peripheral T cells represent, like TcR gamma/delta+ lymphocytes, a new T cell subset physiological role whose remains to be defined.