Antifreeze protein (HPLC-2) from ocean pout was purified from serum using column chromatography on Sephadex G75 and reverse-phase HPLC columns. Single crystals were grown by batch methods at 4 degrees C from a 1.5 M solution of ammonium sulphate (pH 7.1). The crystals diffracted to about 2.5 A resolution at 4 degrees C and belong to the monoclinic space group P2(1), with cell parameters: a = 39.77 A, b = 58.51 A, c = 30.27 A, beta = 102.28 degrees, with two molecules of 6000 M(r) per asymmetric unit.