At the Hadassah University Hospital, Mt. Scopus, Jerusalem, the frequency of patients with relatively penicillin-resistant pneumococci (RPRP) isolates has increased from 0.9 to 10.8% during the years 1979-82. Infants and children were particularly involved. Significantly more RPRP isolates were found in those less than 14 years old than in those who were older (P less than 0.005). The determination of susceptibility or relative resistance to penicillin was based on the disk sensitivity method, which remained unchanged throughout the study period. The minimal inhibitory concentration (MIC) to penicillin G was also determined for 20 RPRP isolates and was found to be in the range of relative resistance to penicillin (0.25 to 0.50 micrograms/ml) in all 20 isolates. A case-control study of 16 index patients examined antibiotic usage during the 60 days preceding pneumococcal isolation. Total antibiotic usage was high in both groups (18.8 vs. 8.8 days, P = 0.2); beta-lactam antibiotic usage was significantly higher in the RPRP group than in the control group (13.3 vs. 4.2 days, 0.01 less than P less than 0.02). General prescribing practices, even in nonisolated areas where there is no need for public health programs to dispense prophylactic antibiotics, may produce sufficiently high antibiotic exposures to aid the emergence of RPRP strains.