A ternary electron transfer protein complex has been crystallized and a preliminary structure investigation has been carried out. The complex is composed of a quinoprotein, methylamine dehydrogenase (MADH), a blue copper protein, amicyanin, and a c-type cytochrome (c551i). All three proteins were isolated from Paracoccus denitrificans. The crystals of the complex are orthorhombic, space group C222(1) with cell dimensions a = 148.81 A, b = 68.85 A, and c = 187.18 A. Two types of isomorphous crystals were prepared: one using native amicyanin and the other copper-free apo-amicyanin. The diffraction data were collected at 2.75 A resolution from the former and at 2.4 A resolution from the latter. The location of the MADH portion was determined by molecular replacement. The copper site of the amicyanin molecule was located in an isomorphous difference Fourier while the iron site of the cytochrome was found in an anomalous difference Fourier. The MADH from P. denitrificans (PD-MADH) is an H2L2 hetero-tetramer with the H subunit containing 373 residues and the L subunit 131 residues, the latter containing a novel redox cofactor, tryptophan tryptophylquinone (TTQ). The amicyanin of P. denitrificans contains 105 residues and the cytochrome c551i contains 155 residues. The ternary complex consists of one MADH tetramer with two molecules of amicyanin and two of c551i, forming a hetero-octamer; the octamer is located on a crystallographic diad. The relative positions of the three redox centers--i.e., the TTQ of MADH, the copper of amicyanin, and the heme group of c55li--are presented.