TMAO-ase is an enzyme of great economic significance in the fish industry since, even below the freezing point, it can produce large amounts of formaldehyde. During frozen storage the resulting formaldehyde-protein interactions induce deleterious effects on the functional properties of frozen fish minces. Knowledge of the structural and enzyme properties is incomplete at present and produces certain ambiguities. A new procedure for extracting TMAOase from saithe kidney (Pollachius virens (L.)), using a non-denaturing detergent, has been developed. Subsequent anion exchange chromatography separated three independent enzyme fractions, differing mainly in their isoelectric points (5.0, 4.5 and 4.1). The three fractions revealed identical pH optima for activity (pH = 5*10), affinity constants (12 mM) and activation energies (4 kcal mol-'). However, differences were observed concerning some of their physicochemical properties such as T; denaturation and spectrophotometric characteristics (unusual absorption at A = 258 nm, related to the presence of DNA fragments). From a structural point of view, as evidenced by the elution profiles, TMAO-ase activity seems to be constituted of high MW protein groups (20 x 10' and 2 x lo6) closely associated with mixed micelles of phospholipids.