Because of reports documenting the occurrence of pneumococci resistant to penicillin, the Microbiology Resource Committee of the College of American Pathologists studied the accuracy of various Kirby-Bauer disks in identifying a pneumococcal organism known to be clinically resistant to penicillin. The only disk that showed acceptable performance was the 1-microgram oxacillin disk, using the inhibition diameter of 19 mm or less as the criterion for predicting clinical penicillin resistance. It is particularly important to understand that, at present, penicillin disks do not provide acceptable predictive accuracy.