There are at least four different existing or proposed interpretive criteria for the disk diffusion susceptibility testing of ticarcillin and ticarcillin plus clavulanic acid (T/C). To assess these criteria, 570 gram-negative bacillary isolates were tested for susceptibility to ticarcillin and T/C by both disk diffusion and broth microdilution methods. These included 53 strains of the family Enterobacteriaceae selected for ticarcillin resistance and high-level beta-lactamase production. The broth microdilution test results were more influenced by increased beta-lactamase production than were disk diffusion results. In the absence of published data indicating which of the two standardized test methods better predicts clinical response, we conclude that until such data are available the more conservative National Committee for Clinical Laboratory Standards tentative criteria for tests with members of the Enterobacteriaceae are appropriate. Our data do not support the use of separate T/C interpretive criteria for Pseudomonas spp. and members of the Enterobacteriaceae. The appropriateness of different interpretive criteria needs further evaluation.