A blue copper protein (Mr 12,000) was purified from cells of "Achromobacter cycloclastes" grown as a denitrifier. When reduced, the blue copper protein transferred electrons to the copper protein nitrite reductase purified from the same cells, whereas a variety of cytochromes from denitrifiers failed to do so. Inclusion of a protease inhibitor, phenylmethylsulfonyl fluoride, in the buffers employed during preparation yielded purified blue copper protein with 18 more amino acid residues and two times more specific enzyme activity than other researchers have found.