In this study, we determined the in vitro inhibitory effects of ceftriaxone sodium, imipenem and ornidazole on hydratase and esterase activities of human erythrocyte carbonic anhydrase-I and II isozymes (CA I and II). Human erythrocyte CAI and II isozymes were purified by Sepharose-4B L-tyrosine affinity chromatography column with a yield of 30% and 40%, a specific activity of 920 and 8,000 EU/mg protein, respectively. In the overall purification procedure, human carbonic anhydrase (hCA)-I and (hCA)-II were purified 104 and 900-fold, respectively. In order to determine the purity of the enzymes, SDS-PAGE was performed. Inhibitory effects of the drugs on hCA-I and hCA-II were determined by using colorimetric method for CO2-hydratase activity assay and spectrophotometric method for esterase activity assay. P-Nitrophenyl acetate was used as a substrate in the spectrophotometric esterase activity assay. The obtained IC50 values (inhibitor concentrations which cause 50% inhibition of in vitro enzyme activity) for esterase activity were 1.900, 0.008, 0.318 mM for hCA-I and 2.542, 0.0258, 0.343 mM for hCA-II for ceftriaxone sodium, imipenem and ornidazole, respectively. IC50 values for CO2-hydratase activity were 0.864, 0.00354, 0.131 mM for hCA-I and 1.118, 0.0214, 0.263 mM for hCA-II for ceftriaxone sodium, imipenem and ornidazole, respectively. In conclusion, ceftriaxone sodium, imipenem and ornidazole showed inhibitory effects on human erythrocte carbonic anhydrase-I and II isozyme activities under in vitro conditions.