Purified recombinant HIV-1 p17 matrix protein significantly increased HIV-1 replication in preactivated peripheral blood mononuclear cell cultures obtained from healthy donors. Because HIV-1 infection and replication is related to cell activation and differentiation status, in the present study, we investigated the role played by p17 during the process of T cell stimulation. Using freshly isolated peripheral blood mononuclear cells, we demonstrate that p17 was able to enhance levels of tumor necrosis factor alpha and IFN-gamma released from cells stimulated by IL-2. IL-4 was found to down-regulate IFN-gamma and tumor necrosis factor alpha, and p17 restored the ability of cells to produce both cytokines. The property of p17 to increase production of proinflammatory cytokines could be a mechanism exploited by the virus to create a more suitable environment for HIV-1 infection and replication. Our data show that p17 exerts its biological activity after binding to a specific cellular receptor expressed on activated T lymphocytes. The functional p17 epitope involved in receptor binding was found to be located at the NH(2)-terminal region of viral protein. Immunization of BALB/c mice with a 14-aa synthetic peptide representative of the HIV-1 p17 functional region (SGGELDRWEKIRLR) resulted in the development of p17 neutralizing antibodies capable of blocking the interaction between p17 and its cellular receptor. Our results define a role for p17 in HIV-1 pathogenesis and contribute to our understanding of the molecular mechanism of HIV-1 infection and the development of additional antiviral therapeutic strategies.