With the demonstration that interleukin 12 can enhance natural killer (NK) cell activity and drive CD4+ lymphocytes toward T helper type 1 (Thl) responses, there is a strong rationale for exploring the use of this cytokine as an immunomodulatory therapy in HIV-1-infected individuals. To assess its potential safety and effects on both immune and virologic aspects of HIV-1 infection, recombinant human IL-12 (rhIL-12) was assessed in rhesus monkeys chronically infected with the simian immunodeficiency virus of macaques (SIVmac). The activity of rhIL-12 on rhesus monkey lymphocytes was confirmed with the demonstration that peripheral blood lymphocyte lysis of the NK-sensitive cell line Colo was enhanced by this recombinant cytokine. Further, rhIL-12 was shown to induce interferon-gamma production by rhesus monkey lymphocytes in vitro. Then, in separate studies, two treatment regimens of rhIL-12 were assessed in SIVmac-infected monkeys: a low-dose regimen (0.1 microg/kg, daily for 4 weeks) and a high-dose regimen (2.5 microg/kg, every 3-4 days, for 3 weeks). Both rhIL-12 treatment regimens were well tolerated by these virus-infected animals. The high-dose regimen of rhIL-12 induced transient decreases in circulating lymphocytes in the SIVmac-infected monkeys. Furthermore, no changes in lymphocyte-associated SIVmac DNA or SIVmac plasma RNA levels were seen in the treated monkeys. These studies indicate that short-term treatment with rhIL-12 is well tolerated and causes no measurable changes in virus load in chronically SIVmac-infected rhesus monkeys.