We prepared several types of derivatives of thielocin B3, a very potent naturally occurring inhibitor for human nonpancreatic secretory PLA2 (sPLA2-II), and conducted a structure-activity relationship study to identify potent sPLA2-II inhibitors with the aim of developing antiinflammatory drugs. The total number of aromatic rings is critical for sPLA2-II inhibition, and the best result was obtained in the case of six rings. The structure of the central part of the inhibitors was not specific, and potent inhibitors were found among the sulfide, sulfone, ether, methylene, and amino derivatives. Although a diester of the terminal carboxylic acid lost its inhibitory activity, having both of the carboxylic acids was not necessary for expression of activity, as illustrated by a glycine derivative with the benzyl ester group 36. Among the newly synthesized derivatives, 18, 20, 29, and 36 showed very potent human sPLA2-II inhibitory activity comparable to that of natural thielocin B3. Their IC50 values are in the range 0.069-0.14 microM, and they are a class of compounds showing the most potent sPLA2-II inhibition to date.