Secondary sulfonamides (4a-8h) incorporating acetoxybenzamide, triacetoxybenzamide, hydroxybenzamide, and trihydroxybenzamide and possessing thiazole, pyrimidine, pyridine, isoxazole and thiadiazole groups were synthesized. Lactoperoxidase (LPO, E.C.126.96.36.199), as a natural antibacterial agent, is a peroxidase enzyme secreted from salivary, mammary, and other mucosal glands. In the present study, the in vitro inhibitory effects of some secondary sulfonamide derivatives (4a-8h) were examined against LPO. The obtained results reveal that secondary sulfonamide derivatives (4a-8h) are effective LPO inhibitors. The K-i values of secondary sulfonamide derivatives (4a-8h) were found in the range of 1.096 x 10(-3) to 1203.83 mu M against LPO. However, the most effective inhibition was found for N-(sulfathiazole)-3,4,5-triacetoxybenzamide (6a), with K-i values of 1.096 x 10(-3) +/- 0.471 x 10(-3) mu M as non-competitive inhibition.