Sixty-three patients that have been splenectomized for various disorders were vaccinated with Pneumovax 23, the currently available pneumococcal vaccine. Before and one month after splenectomy, IgG antibodies against 8 pneumococcal polysaccharide antigens (types 3, 4, 6B, 7F, 10A, 14, 19F, and 20) were determined by a highly reproducible and type-specific enzyme-linked immunosorbent assay (ELISA). In order to increase the specificity of the assay, this method involved the use of a capture antibody (F(ab')2-fragments of a type-specific rabbit antipneumococcal hyperimmune serum) to bind the pneumococcal antigens to the plastic surface of microtiter plates and the absorption of these sera with the cross-reacting C polysaccharide. Rates of patients showing a two-fold antibody increase were dependent on pneumococcal type, ranging from 12.7% to 33.3%. Antibody responses of single patients were not uniform for all pneumococcal serotypes investigated. Only one patient responded to all of the eight antigens tested. In spite of relatively low response rates, splenectomized patients should be routinely vaccinated with the pneumococcal vaccine, especially when the low rate of adverse reactions is taken into consideration. It is emphasised that the results of the present study and those reported in the literature have to be compared and interpreted with caution, because the available data on the antibody response to pneumococcal vaccination are based on assays that differ substantially in methodology.