Constitutive expression of alpha(1)-antitrypsin (alpha(1)AT), a serine protease inhibitor, by a recombinant simian virus-40-based vector blocks both HIV gp160 and p55 processing, and so is a powerful inhibitor of HIV replication. To apply these findings more effectively in devising HIV therapies, we tested HIV LTR conditional promoter, to drive the expression of alpha(1)AT. SV[LTR](AT) was designed so that synthesis of human alpha(1)AT would be trans-activated by HIV infection. Cell lines and primary human lymphocytes were transduced with SV[LTR](AT) without selection and detectable toxicity. Responsiveness of alpha(1)AT expression to HIV Tat or HIV challenge was confirmed by Northern blotting, RT-PCR, cytofluorimetry and immunostaining. SV[LTR](AT)-transduced cells were protected from HIV-1(NL4-3) at a challenge dose of 0.04 MOI (T-cell lines) or 0.2 MOI (peripheral blood lymphocytes). Conditional expression of alpha(1)AT consistently protected T cells from HIV challenge as effectively as did constitutive expression. Combining the efficiency of rSV40 vectors with HIV-responsive expression of a highly effective anti-HIV therapeutic may be an effective approach to gene therapy of HIV replication.