PCP vaccine was administered to 21 adult hemodialysis patients and 25 control patients with no renal nor hepatic dysfunction. Antibody concentrations measured by the RIA technique were lower in the hemodialysis than in the control patients in both prevaccination sera and specimens obtained 3 to 4 weeks following vaccination. However, 6 months after vaccination, RIA GMACs in hemodialysis and control subjects were more nearly alike. OTs against Streptococcus pneumoniae, types 1 and 3, were measured in paired sera from vaccine recipients. Opsonic activity against S. pneumoniae, type I, was detected as frequently in the 3 to 4 week postvaccine sera of the dialysis as in those of control patients. However, there was a poor correlation between OTs and RIA-ACs against this organism. There was good correlation between OT and RIA-AC against type 3 pneumococci in 3 to 4 week postvaccine sera, and significantly more control patients had opsonic activity detectable in these specimens. No clinical S. pneumoniae infections have been observed in vaccine recipients. However, continued follow-up will be necessary to document the degree to which hemodialysis patients are protected by pneumococcal vaccines and the relationship between protection and antibody concentration or OTs.