We investigated the effects of the antibiotic drugs cefaperazone/sulbactam and ampicillin/sulbactam on the in vitro activity of the enzyme glucose-6-phosphate dehydrogenase (G6PD). The enzyme was purified from human erythocytes using 2',5' ADP-Sepharose 4B affinity gel. The enzymatic activity was measured spectrophotometrically at 340 nm, according to the method of Beutler. The I50 values were determined from Activity % - [Drug] graphs, and the Ki constants and inhibition types for each drug were determined using Lineweaver-Burk graphs. The I50 value was 13.5 mg/ml for cefaperazone/sulbactam and 36 mg/ml for ampicillin/sulbactam. The Ki constants were 10.16 for ampicillin/sulbactam and 38.22 for cefaperazone/sulbactam. Cefaperazone/sulbactam competitively inhibited G6PD activity, whereas ampicillin/sulbactam non-competitively inhibited the activity of the enzyme.