This research addressed the effects of two prominent nitrogen-containing aromatic compounds (NCACs), quinoline and 4-azafluorene, on respiratory electron transport (ET) in Escherichia coli. ET was estimated spectrophotometrically using reduction rates of iodonitrotetrazolium chloride (INT), which is reduced in vivo to a red colored formazan (INTF). It was noted that both NCACs gave anomalous dose-response behavior in INT assays: in a defined “threshold” dose range, INT reduction rates near or above the controls were observed. Comapred with controls and low doses, the threshold doses for the NCACs showed different INT reduction kinetics, decreased cellular oxygen consumption, and decreased viable cell densities. These observations and experiments with E. coli spheroplast preparations, gram positive cells, and deep rough mutants supported the hypothesis that the NCACs caused removal of outer membrane constituents and probably interference with cell membrane function. Data from the INT bioassays, comparative oxygen demand studies, assays of INT response in bacteria with different outer membrane characteristics, and transmission electron microscopy are presented in support of this hypothesis.