Interleukin-4 is implicated in the pathogenesis of HIV-induced AIDS, and causes enhancement of replication of virus strains that utilize the CXCR4 (X4) co-receptor. In this study, we explored the effects of IL-4 antisense (AS) DNA on replication of X4, simian human immunodeficiency viruses, SHIVKU-2 and SHIV89.6P. AS IL-4 oligomer caused inhibition of virus replication in cultures of CD4(+) T cells and macrophages derived from macaques. Plasmid expressing AS IL-4 DNA was also effective in abrogating virus replication in macrophage cultures. Relevance of these cell culture studies was confirmed in vivo by treating SHIV89.6P-infected macaques with AS IL-4 DNA. Six macaques were inoculated with the virus and four were treated with AS IL-4 DNA. This resulted in a significant decrease in viral RNA concentrations in the liver, lungs and spleen, tissues that are all sites of virus replication in macrophages. This is the first demonstration of effective inhibition of an HIV-like virus in tissues by AS DNA of a cytokine. In the present era of increasing resistance of HIV to antiviral compounds, exploration of adjunct therapies directed at host responses in combination with antiretroviral drugs may be of value for the treatment of AIDS.