Successful biologic treatment of oil shale retort wastewaters depends partly on the biooxidation of aromatic amines, compounds reported to resist mineralization. To investigate possible causes of recalcitrance, we followed the disappearance of selected aromatic amines in both single and mixed-substrate systems to model the more complex arrays found in retort waters. Biodegradability of each of 35 aromatic amines was evaluated in a series of batch, dispersed phase cultures with each compound as the sole source of carbon, nitrogen, and energy. Of these aromatic amines, 15 u.v. absorbance. After 1 to 3 months of exposure to acclimated cultures, none of the other 20 aromatic amines was biodegraded. An equal mass mixture of 12 biodegradable aromatic amines was mineralized in one week by the pooled, individually acclimated cultures. This mixture appeared to be less degradable, however, when added to a retort water. While inhibitory effects of the retort water may have accounted for the resistance of some of the endogenous aromatic amines, it was likely that the majority remaining after biooxidation were inherently recalcitrant.