Methylamine oxidase from Arthrobacter P1 was purified to homogeneity. The enzyme oxidizes primary amines but not tyramine or polyamines like spermine and putrescine. The enzyme activity has a pH optimum of 8.0 with methylamine, and is inhibited by certain cations as well as anions at rather low concentrations. The enzyme has an Mr of 167900, an isoelectric point of 4.6, consists of two (probably identical) subunits (Mr 82250) and contains two copper atoms but no sugar residues. The visible absorption spectra of the enzyme as it is isolated (broad maximum at 480 nm), that of its reduced form obtained on addition of excess of methylamine (maximum at 470 nm) and that of phenylhydrazine-inhibited enzyme (maximum at 440 nm) are very similar to those of eucaryotic copper-containing amine oxidases (EC 1.4.3.6). Also the stoichiometry of inhibition with carbonyl group reagents is similar since the enzyme reacted with only one methylhydrazine. The adduct isolated from copper-free enzyme, treated with 2,4-dinitrophenylhydrazine, was identical to that found in bovine serum amine oxidase treated with this compound after copper removal. This indicates that the enzyme is a copper-quinoprotein amine oxidase, the first example from bacterial origin.