Since H. influenzae immunization has been introduced S. pneumoniae has become the most common invasive bacterial pathogen in children. The increased incidence of pneumococcal diseases has been complicated by the appearance of multidrug resistant (MDR) isolates and low levels of serotype-specific antibodies in young children. We have recently reported an MDR serotype 14 (S14) organism isolated from a child who had previously carried a fully susceptible S14 as well as an MDR S23F strain. Antibiotic resistance pattern, electrophoresis of penicillin-binding proteins(PBPs) and pulsed-field gel electrophoresis (PFGE) of chromosomal DNA indicated that the MDR S14 organism was very similar to the MDR S23F strain, suggesting serotype transformation in vivo. The DNA sequences of PBP genes can be used to determine relatedness of various pneumococcal isolates. In this work we determined the DNA fingerprints of PBP 2x and 2b genes of the MDR S14 and the MDR S23F organism by digestion of PCR amplified fragments with restriction enzymes ([Illegible Text], [Illegible Text] and MseI+DdeI), followed by end-labeling, separation on a polyacrylamide gel and autoradiography. The S14 and S23F isolates described above carry the same PBP 2x and 2b genes, indicating that the genes for capsule biosynthesis were horizontally transferred between strains. The same kind of capsular transformation may be the explanation for a S9N isolate, infecting a patient in a New York City hospital, which shared the same antibiotype, PFGE and PBP 2x and 2b DNA fingerprints as 23F isolates commonly found in that hospital. Furthermore, the same mechanism may apply to five isolates of S23F and five isolates of S19F causing infections in a South Korean hospital that again share the same antibiotype, PFGE pattern and the identical PBP 2x and 2b DNA fingerprints. This work provides further evidence for pneumococcal capsular transformation in vivo, most likely by horizontal transfer of genetic information encoding for capsular serotype. Our findings suggest that the widely spread epidemic clone of 23F MDR S. pneumoniae may be a frequent recipient in such genetic events. Since the nasopharynx of young children, especially children attending day care centers, is frequently colonized by more than one serotype, capsular transformation may be a frequent mechanism by which S. pneumoniae avoids serotype-specific opsophagocytic antibodies during infection.