Earlier experiments in Trypanosoma cruzi-infected rats showed that recombinant rat (Rr) interferon (IFN)-gamma given shortly after infection ameliorated acute disease without modifying the serum titers of endogenously synthesized IFN-gamma and tumor necrosis factor. To gain some insight into the processes underlying this protective effect, 21-day old 'I' rats that were infected with T. cruzi and the following day started with a 20-day cycle of RrIFN-gamma injections (20000 IU/rat/day) were investigated for the in vitro replication of T. cruzi and nitric oxide (NO) production by peritoneal macrophages (day 7 post-infection, pi), antibodies with lytic activity against T. cruzi (days 7, 20, and 28 pi), and serum levels of biologically active interleukin (IL)-6 (days 15 and 30 pi). Therapy with RrIFN-gamma rendered cultured peritoneal macrophages less permissive to infection with T. cruzi. Such an effect was not accompanied by higher amounts of NO in macrophage cultured supernatants, compared with those from T. cruzi-infected rats receiving no RrIFN-gamma, which appeared not to be protected from in vitro infection. Acutely T. cruzi-infected rats had significant amounts of IL-6 in their sera - this not being the case in infected rats given RrIFN-gamma, whose levels appeared decreased as in control rats. The presence of complement-mediated anti-T. cruzi lytic antibodies was not modified by RrIFN-gamma. Likewise, heart histology at day 7 pi revealed that treatment with RrIFN-gamma made no differences as to the amount of acute inflammation, but tended to reduce the myocardial parasite load.