Levels of nitrate reductase (NR) protein in Hansenula anomala and Hansenula wingei were determined using specific antiserum raised against the enzyme from H. anomala. Extracts from nitrate-grown cells contained NR protein, while in those from cells grown on ammonium, glutamine or peptone, no cross-reacting material could be observed. Enzyme activity correlated with the levels of cross-reacting material. When nitrate was used as nitrogen source, NR was always present, even in cultures with ammonium, glutamine or peptone, although in these cases both the levels of activity and protein were lower. NR activity was consistently two to four times higher in cells grown in glucose than in cells grown in ethanol. Nitrate was required for NR induction, and deprivation of nitrate from nitrate-grown cells resulted in a rapid loss of NR activity.