The individual members of a homologous series of phenoxazone ethers related to ethoxyresorufin were O-dealkylated, and the parent compound phenoxazone was ring-hydroxylated, each at different rates with hepatic microsomes of untreated rats. A structure-activity relationship (SAR) was plotted, relating the rate of O-dealkylation to the length and type of the ether side-chain. Phenobarbitone (PB), 3-methylcholanthrene (MC), Aroclor 1254 (ARO), isosafrole (ISO) and SKF-525A each induced preferentially the O-dealkylation of different members of the homologous series, resulting in the appearance of 5 different SAR plots, which characterized and differentiated between the 5 different inducers. beta-Napthoflavone (BNF) had a similar effect to MC, whereas pregnenolone 16 alpha-carbonitrile treatment caused no large change in the metabolism of any of the substrates tested. For characterizing the effects of the different inducers it was largely sufficient to compare the O-dealkylations of just 4 of the ethers: methoxy-, ethoxy-, pentoxy- and benzyloxyphenoxazone. Very high degrees of induction were seen. MC and ARO each induced preferentially the O-dealkylation of ethoxyphenoxazone (51- and 61-fold respectively). PB and SKF-525A each induced preferentially the O-dealkylation of pentoxyphenoxazone (283- and 324-fold respectively). ISO induced preferentially the O-dealkylation of benzyloxyphenoxazone (43-fold). For any particular induced type of microsomes the substrate with the fastest metabolism was not necessarily the substrate whose metabolism was induced the most, so that in order to characterize each of the 5 different inducers (PB, MC/BNF, ARO, ISO, SKF) it was necessary to compare both the degrees of induction and the specific activities of the reactions. Experiments with purified cyt. P-450 isozymes showed that ethoxyphenoxazone and pentoxyphenoxazone were highly selective substrates for the major isozymes induced by MC and PB respectively, whilst benzyloxyphenoxazone was a good substrate for both isozymes. Experiments using the organic inhibitors metyrapone and alpha-naphthoflavone and inhibitory antibodies against individual cyt. P-450 isozymes indicated that similar substrate selectivities occurred with the monooxygenase system in the microsomal membrane. It is suggested that the use of some or all of these homologous phenoxazone ethers will provide both a simple routine test for the characterization of several types of inducing agents and a powerful tool for investigating the biochemical basis for cyt. P-450 isozyme substrate selectivity.