Plant activation of three isomers of phenylenediamine o- m- and p-phenylenediamine, has been studied. Two in vitro plant systems have been used: Persea americana S117 with mixed-function oxidase (MFO) and peroxidase activities, and Zea mays S9 which contains only peroxidase activity. As genetic endpoint, the classical Salmonella tester strains. TA98 and TA100, their derivatives with high O-acetyltransferase levels (YG1024 and YG1029, respectively) and TA98/1.8-DNP6, deficient in this enzyme, have been assayed. Of the three isomers studied, only m-PDA was activated to mutagenic product(s) by both plant systems. This activation required the bacterial O-acetyltransferase activity to give frameshift mutagenic product(s), detected in TA98 and YG1024 strains. In all the assays the P americana system was more potent than the Z. mays system in activating m-PDA. A slight increase of the number of YG1029 revertants was detected when m-PDA was activated by P. americana, suggesting that this compound can be also converted into ultimate mutagenic product(s) that induce base-pair substitutions. m-PDA activation by Z. mays was dependent on the peroxidase activity of this system, but the activation produced by P. americana was totally dependent on MFOs, because a total inhibition of the mutagenic response was found when these activities were inhibited. In addition, the P. americana system was more potent in generating proximal mutagenic forms from m-PDA than S9 from non-induced rat liver, although S9 from Aroclor 1254-induced Sprague-Dawley male rats was the most potent system in the m-PDA activation. These results indicate that the P. americana system can be useful in determining the role of mixed-function oxidases in plant activation of xenobiotics.