Patients with bacteremia, bacterial endocarditis, or acquired immunodeficiency syndrome (AIDS) were prospectively studied using monoclonal antibody reagents to assess alterations in T-lymphocyte subpopulations. Patients with endocarditis had significantly higher ratios of T-helper (OKT4+) to T-suppressor-cytotoxic (OKT8+) cells than did patients with bacteremia alone. Staphylococcus aureus endocarditis patients had a mean ratio of 8.49 (range 4.73-22.36) while S aureus bacteremia had a mean ratio of 2.75 (range 2.15 to 3.21). Similar results were found with Staphylococcus epidermidis endocarditis (mean 1.62) and bacteremia (mean 1.23). Klebsiella pneumoniae endocarditis (5.10) and sepsis (4.32), and E coli bacteremia (2.15). Nine male patients with AIDS had markedly depressed ratios (mean 0.25, range 0.04 to 0.67) while eight male homosexuals with unexplained lymphadenopathy ("pre-AIDS") had normal or increased ratios. Bacteremic infections are associated with an increased OKT4+/OKT8+ ratio with the degree of increase dependent upon virulence, location, and duration of infection. The immunomodulating effects of infection are manifested in changes in T-cell subsets, and these measurements can be useful in clinical management.