Three bacterial species, i.e., Ralstonia sp. SJ98, Arthrobacter protophormiae RKJ100, and Burkholderia cepacia RKJ200, have been examined for their efficiency and kinetics behavior toward PNP degradation. All the three bacteria utilized PNP as the sole source of carbon, nitrogen, and energy. The rates of radiolabeled [U-C-14]PNP degradation by all the bacteria were higher in the nitrogen-free medium compared to the medium with nitrogen. The apparent K-m values of PNP degradation by SJ98, RKJ100, and RHJ200 were 0.32, 0.28, and 0.23 mM, respectively, as determined from the Michaelis-Menten curves. The maximum rates of PNP degradation (V-max) according to Lineweaver-Burk's plots were 11.76, 7.81, and 3.84 mu mol PNP degraded/min/mg dry biomass, respectively. The interpretation drawn from the Lineweaver-Burk's plots showed that the PNP degradation by SJ98 was stimulated by 4-nitrocatechol and 1,2,4-benzenetriol. Benzoquinone and hydroquinone inhibited PNP degradation by RKJ200 competitively and competitively, respectively, whereas in the case of RKJ200, benzoquinone and hydroquinone inhibited PNP degradation in an uncompetitive manner. beta-Ketoadipate did not affect the rate of PNP degradation in any case.