A series of 2-[4-(thiazol-2-yl)phenyl]propionic acids substituted at various positions were propared by the reaction of diethyl 2-methyl-2-(4-thiocarbamoylphenyl)malonates with α-bromoaldehyde diethyl acetals or α-haloketones followed by hydrolysis of esters. The inhibition of prostaglandin H synthetase (cyclooxygenase) was assayed by use of an enzyme preparation from guinea pig polymorphonuclear leukocytes. Examination of the structure-activity relationship of these compounds indicated that the subsititution pattern with halogens at position 3 (R₁) of the benzene ring and a methyl group in position 4 (R₂) and/or 5 (R₃) of the thiazole ring were favorable for inhibitory activity. The compounds bearing bulky alkyl or polar functional groups at the R₂ position were weak inhibitors. The potent inhibitors of cyclooxygenase were tested for their ability to reduce carrageenin-induced inflammation of rat paws. These derivatives had strong anti-inflammatory activity based on their strong inhibition of cyclooxygenase, with some exceptions, including those with a thiomethyl group at R₁.