The immune response to intravenously administered bacteriophage ΦX 174 and subcutaneously administered tridecavalent pneumococcal polysaccharide vaccine was studied in 31 patients with anatomical or functional asplenia. Antibody responses to primary immunisation with ΦX 174 were significantly decreased while clearance was normal. Secondary responses were quantitatively normal; however, production of antibody did not switch from IgM to IgG as seen in controls. All groups of asplenic patients except those patients with Hodgkin's disease demonstrated significant seroconversions in response to pneumococcal polysaccharide antigens. One patient with Hodgkin's disease, treated with local irradiation only, demonstrated normal responses to pneumococcal capsular antigens. 10 of the 12 capsular antigens for which antibody was measured stimulated threefold increases in antibody in the 26 asplenic patients without Hodgkin's disease, which is similar to that observed in controls. Since the majority of cases of overwhelming postsplenectomy infection are caused by Streptococcus pneumoniœ,all patients with either anatomical or functional asplenia should receive pneumococcal polysaccharide vaccine.