Some immunologic changes in patients with infective endocarditis were measured during therapy with antibiotics. T cells and C3 levels were low in 29 and 59% of the subjects, respectively. The circulating immune complexes were high and showed a negative correlation with serum C3 levels. Teichoic acid antibodies with titer of above 5 were generally seen only in cases of bacteremia in which endothelial damage had taken place. The antibody was specific for gram-positive organisms because it was negative in gram-negative bacteremia. The antibody showed a fall during treatment and hence could be used to monitor efficacy of antibiotic therapy.