Two features of simian immunodeficiency virus (SIV) infection are emphasized: a transitory decrease in CD4 T cells in the first 2 weeks of infection followed by CD8 T-cell rise, and immune cell activation occurring by 4 weeks and persisting throughout the illness. The short-term changes included a fall in CD4 T cells by 2 weeks with partial recovery by 4 weeks and a CD8 rise that starts at 2 weeks. Subsequent characterization of CD4 T cells showed reduced expression of HLA-DR and CD25 (IL-2 receptor alpha chain) antigens later in SIV infection. Immune cell activation is evident in increased serum levels of neopterin and soluble CD8 antigen. Serum β2-microglobulin changes are less marked. Activation of CD8 T cells is reflected by increased percentages of cells expressing HLA-DR antigen. The B-cell numbers increased late in the course of SIV infection. Increased expression of the CD78 (Leu 21) activation phenotype was also seen in some monkeys. The immune activation changes (serum neopterin levels) induced by SIV infection in rhesus macaques appear to be associated with duration of illness, although the number of monkeys observed until death were too few for conclusive data. Thus, immune activation as well as T-cell deficiency may reflect significant immunopathogenic processes in SIV-induced disease.