The purification of two soluble CO-binding cytochromes c from Nitrosomonas europaea is described. Cytochrome cCO-550 ran on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis with an apparent Mr of 32 000, whereas for cytochrome cCO-552 the apparent Mr was 16 000. Redox potentials (Em, 7) were determined as +140 and -50mV respectively. Cytochrome cCO-550 was co-purified with a cytochrome c-553, for which an unusually high redox potential of +450mV was measured. These latter components were not resolved by gel-filtration chromatography or electrophoresis under denaturing conditions.