Upon transfer into a mineral NO2- medium, the activity of the NO2--oxidizing system in cells of N. agilis K1 is dependent on the conditions of the previous heterotrophic growth. The NO2--oxidizing system is inducible. The PHB [poly(b-hydroxybutyric acid)] content depends on the exogenous N source. Cells grown with NO3- possess a high, with NO3- an intermediate, and with NH3, a low content of PHB. The activity of the NO2- oxidn. in heterotrophically grown cells also varies with the exogenous N source. In comparison with the activity of lithoautotrophically grown cells (100%), yeast ext.-peptone-grown cells possess 28, NO3- cells 20, and NH3 cells 2% of the NO2- oxidn. activity. The time for max. induction of the NO2- oxidizing system varies in a similar way when cells are grown in medium with NO3-, NO2-, and (NH4)2SO4. The highest activity is measured after 27, 33, and 120 h, resp. Induced cells degrade PHB and cellular protein.