The anthropogenic chemicals nonylphenol, bisphenol A, phthalic acid benzyl n-butyl ester, phthalic acid di-n-butyl ester and phthalic acid di(2-ethylhexyl) ester have been shown to possess sex hormone-like activity. To explore the possible actions of these chemicals on the autacoid synthesis in the body, we investigated the effects of nonylphenol, bisphenol A, phthalic acid benzyl n-butyl ester, phthalic acid di-n-butyl ester and phthalic acid di(2-ethylhexyl) ester on the activities of cyclooxygenase-1 and -2. Bisphenol A and all three phthalic acid derivatives had no significant effect on the cyclooxygenase-1 and -2 activities up to 100 microM. On the other hand, nonylphenol exhibited a marked inhibition on the cyclooxygenase-1 activity (10-100 microM nonylphenol, 7-95% inhibition), with no detectable change in the activity of cyclooxygenase-2. The inhibition patterns for the substrate, arachidonic acid, and a cofactor, phenol, were competitive and uncompetitive, respectively. These results suggest that nonylphenol can be a selective inhibitor of cyclooxygenase-1 activity.