Three infectious, attenuated molecular clones of simian immunodeficiency virus (SIVmac) were tested for viral and host determinants of protective immunity. The viruses differed in degree of virulence from highly attenuated to moderately attenuated to partially attenuated. Levels of immune stimulation and antiviral immunity were measured in rhesus macaques inoculated 2 years previously with these viruses. Monkeys infected with the highly attenuated or moderately attenuated viruses had minimal lymphoid hyperplasia, normal CD4/CD8 ratios, low levels of SIV-specific antibodies, and cytotoxic T-lymphocyte activity against p55gag (Gag) or gp160env (Env). Monkeys infected with the partially attenuated virus had moderate to marked lymphoid hyperplasia, normal CD4/CD8 ratios, high levels of SIV-specific antibodies, and cytotoxic T-lymphocyte activity against both Gag and Env. After pathogenic virus challenge, monkeys immunized with the partially attenuated virus had 100- to 1,000-fold-lower viral load in peripheral blood mononuclear cells and lymph node mononuclear cells than naive control animals. One of four monkeys immunized with the highly attenuated virus and two of four monkeys immunized with the moderately attenuated virus developed similarly low viral loads after challenge. These three attenuated strains of SIV induced a spectrum of antiviral immunity that was inversely associated with their degree of attenuation. Only the least attenuated virus induced resistance to challenge infection in all immunized monkeys.