Transformation studies were carried out with a penicillin susceptible (MIC 0.006 mg/l) laboratory strain of Streptococcus pneumoniae as recipient. Donor DNA was prepared from two clinical isolates of S. pneumoniae, four isolates of S. mitior and five isolates of S. sanguis. DNA from both isolates of S. pneumoniae generated penicillin-resistant transformants (MICs 0.03-2.0 mg/l). In addition, one isolate each of S. mitior and S. sanguis transformed the recipient to increased penicillin resistance, with MICs of 0.125 mg/l. DNA from the S. sanguis strain generated a transformant which showed a reduction in the penicillin-binding affinity of penicillin-binding protein (PBP) 2B. DNA from a second S. mitior strain generated intermediately resistant (MIC 0.5 mg/l) transformants in a single transformation round. The PBP profiles of these transformants showed an apparent molecular size alteration of PBP 2B, corresponding to a PBP found in the donor strain, and PBP 1B was no longer detected. Isolates of S. pneumoniae may apparently develop penicillin resistance either intrinsically, by intragenetic transfer, or by transfer of penicillin resistance determinants from viridans streptococci.