Ten patients with symptomatic HIV infection (six with ARC, four with AIDS) received tetanus and 23-valent pneumococcal vaccination. Anti-tetanus IgG and IgM, and anti-pneumococcal IgG against all 23 capsular types of the vaccine were measured on days 0, 11, 17, 30, and 90. Anti-pneumococcal IgG were simultaneously determined in two plasma pools of 100 healthy unimmunized blood donors and of 112 healthy adults who had previously received a 14-valent pneumococcal vaccination. Peak IgG responses to both vaccines were observed on day 17; thereafter, the antibody levels gradually fell again. Anti-tetanus IgG rose from 0.6 U/ml (geometric mean) to 2.0 U/ml on day 17. Anti-tetanus IgM remained unchanged. Anti-pneumococcal IgG increased only by 1.14-fold compared with pre-vaccination levels (geometric mean of IgG rises against all 23 polysaccharides in 10 patients), and exceeded the upper 95% limit of unvaccinated blood donors in only 30 out of 230 specimens. Pre-vaccination levels for pneumococcal type-specific IgG were significantly higher in HIV-infected patients compared with the pool of unimmunized healthy controls, possibly indicating a higher rate of previous pneumococcal infections in HIV-seropositive subjects. However, post-pneumococcal vaccination levels were significantly lower in HIV-infected patients than in the pool of healthy controls. The increase in anti-tetanus IgG significantly correlated with the level of CD4 lymphocytes and with in vitro lymphocyte proliferation by pokeweed mitogen (5 micrograms/ml) and phytohaemagglutinin (2.5 micrograms/ml), confirming a particularly low vaccination response in patients who were severely immunocompromised.