The susceptibility of 447 clinical bacterial isolates to mezlocillin and carbenicillin was tested by standardized agar disk diffusion and reference broth micro-dilution methods. Tentative interpretive criteria for disk susceptibility testing by using 75 micrograms mezlocillin disks are proposed: susceptible, greater than or equal to 16 mm; indeterminate, 13 to 15 mm; and resistant, less than or equal to 12 mm. These would be applicable to both Pseudomonas species and the Enterobacteriaceae, but not to Staphylococcus aureus. For S. aureus, the breakpoints for susceptible, greater than or equal to 29 mm, and resistant, less than or equal to 28 mm, hold for mezlocillin as well as for the other penicillinase-susceptible penicillins.