To learn the prevalence of penicillin-resistant pneumococci and the distribution of serotypes in proved pneumococcal infections, we studied 98 pneumococci recovered from blood over a 4-year period. Penicillin susceptibility was determined by the agar dilution method. Serotyping was done by the capsular swelling (quellung) test. Only one strain showed diminished susceptibility to penicillin (minimal inhibitory concentration, 0.12 micrograms/ml). Twenty-three different serotypes were identified; types 3, 4, 6, 8, 9, 14, and 19 were the most frequent. Type 4 was the most common serotype. Sixty-two percent of strains were serotypes included in a recently licensed 14-type pneumococcal polysaccharide vaccine (Pneumovax, Merck), and an additional 16% were antigenically related serotypes. Even though penicillin-resistant strains of pneumococci may rarely cause bacteremic pneumococcal infection, we suggest that isolates from cerebrospinal fluid, blood, and other normally sterile body fluids be tested for penicillin susceptibility. Inclusion of additional serotypes in future pneumococcal polysaccharide vaccines should be based on the current distribution of serotypes in large series of proved pneumococcal infections.