Tienilic acid (TA) is metabolized by human liver microsomes in the presence of NADPH with the major formation of 5-hydroxytienilic acid (5-OHTA) which is derived from the hydroxylation of the thiophene ring of TA. Besides this hydroxylation, TA is oxidized into reactive metabolites which covalently bind to microsomal proteins. Oxidation of an isomer of tienilic acid (TAI), bearing the aroyl substituent on position 3 (instead of 2) of the thiophene ring, by human liver microsomes, gives a much higher level of covalent binding to proteins. Both covalent binding of TA and TAI metabolites are almost completely suppressed in the presence of glutathione. These three activities of human liver microsomes (TA 5-hydroxylation, covalent binding of TA and TAI metabolites) seem dependent on the same cytochrome P450 of the IIC subfamily, since (i) antibodies against human liver cytochromes P450 IIC strongly inhibit these three activities, (ii) there is a clear correlation between these activities in various human liver microsomes, and (iii) TA acts as a competitive inhibitor for TAI activation into electrophilic metabolites (Ki approximately equal to 25 microM) and TAI inhibits TA 5-hydroxylation. However cross inhibition experiments indicate that tienilic acid hydroxylation and mephenytoin hydroxylation, a typical reaction of some human liver P450 IIC isoenzymes, are not catalysed by the same member of the P450 IIC subfamily.