The bacterial cultures also were used to study changes in crude oil toxicity as a function of biodegradation activity (Table 3). Inhibition of luminescence in all tested samples were comparable to the background level in the abiotic control that caused from 6 to 9% of luminescence reduction. During the time of the incubation luminescence inhibition slightly increased. This inhibition could be due to the presence of toxic intermediates produced during biodegradation of petroleum hydrocarbons. The samples were not to be found toxic to Vibrio fischeri. Detection of this activity or residual toxicity remaining after biodegradation underscores the need to test for toxicity changes during biodegradation studies.