Five sulfonamides were selected to study for their inhibitory activity against dihydropteroate synthase from Escherichia coli by measuring the minimum inhibitory concentration (MIC), the concentration required for 50% inhibition (I50) and the inhibitor constant (Ki). The MIC value for sulfanilamide was 4.30 x 10-2 mol/l when the LB broth was used as a nutrient medium. The MIC values for sulfanilamide and p-aminobenzenesulfonamidopropyl bromide were 1.31 x 10-3 mol/l and 2.91 x 10-3 mol/l, respectively, when the glucose minimum medium was used as a nutrient medium. The enzyme precipitated with 20-60% saturated ammonium sulfate was used for the determination of I50 and the I50 values for sulfanilamide, N,N'-bis(sulfanilyl)-L-cystine, p-aminobenzenesulfonamidopropyl thiosulfate and 4-(4-aminobenzenesulfonamido) benzenesulfonamido-glycine were found to be 2.15 x 10-3, 3.23 x 10-3, 3.35 x 10-3 and 2.70 x 10-5 mol/l, respectively. The purity of the enzyme was increased 209-fold after the affinity chromatography step, however, the enzyme showed seven bands of proteins by acrylamide gel electro-phoresis. The optimal pH and temperature of the enzyme were 8.58 and 45-46 C, respectively. The Km value for p-aminobenzoic acid was 2.86 x 10-5 mol/l. The Ki values for sulfanilamide, p-aminobenzenesulfonamidopropyl bromide, N,N'-bis(sulfanilyl)-L-cystine, p-aminobenzenesulfonamidopropyl thiosulfate and 4-(4-aminobenzenesulfonamido) benzenesulfonamidoglycine were 4.30 x 10-4, 7.50 x 10-4, 3.50 x 10-4, 5.38 x 10-4 and 7.50 x 10-6 mol/l, respectively. All five sulfonamides were competitive inhibitors with p-aminobenzoic acid.