Inhibitory effects of some antibiotics on glucose-6-phosphate dehydrogenase from the erythrocytes of human have been investigated. For this purpose, at the beginning, erythrocyte glucose-6-phosphate dehydrogenase was purified 13.654 times in a yield of 28% by using ammonium sulphate precipitation and 2′,5′-ADP Sepharose 4B affinity gel. Temperature of +4°C was maintained during the purification process. Enzyme activity was determined with the Beutler method by using a spectrophotometer at 340 nm. This method was utilized for all kinetic studies. Sodium ceftizoxime, sodium ampicillin, sodium cefuroxime, sodium cefazolin, sodium cefoperazone, streptomycin sulphate, gentamicin sulphate, and netilmicin sulphate were used as antibiotics. All the antibiotics indicated the inhibitory effects on the enzyme. K_iconstants for glucose-6-phosphate dehydrogenase were found by means of Lineweaver–Burk graphs. While sodium cefoperazone, gentamicin sulphate, and netilmicin sulphate showed competitive inhibition, the others displayed non-competitive inhibition. In addition, I₅₀values of the antibiotics were determined by plotting activity percent vs [I]. In addition, in vivo studies were done for sodium sefuroxime in Sprague–Dawley type rats. It was found that G6PD in erythrocyte was more inhibited by the drug in 2.5 h. 2000 Academic Press@p$hr