Neuropsin (M-r 25 032) is a serine protease expressed in the limbic system of mouse brain. It has been implicated in various neurological processes including formation of memory and may be important as a drug target in the treatment of epilepsy. The recombinant protein was produced using a baculovirus expression system and was purified. Two crystal forms were obtained by a hanging-drop vapor-diffusion method with polyethylene glycol. Preliminary X-ray crystallographic analysis revealed that; crystal form I belongs to triclinic space group P1 with unit cell dimensions a = 97.16 Angstrom, b = 97.12 Angstrom, c = 46.75 Angstrom and alpha = 99.17 degrees, beta = 99.17 degrees, gamma = 117.35 degrees. Self-rotation function analysis of these data of form I indicates the position of a noncrystallographic threefold axis. There are six molecules in the crystallographic asymmetric unit. Crystal form II also belongs to triclinic space group P1 but has unit cell dimensions of a = 38.40 Angstrom, b = 55.16 Angstrom, c = 65.37; Angstrom and alpha = 95.38 degrees, beta = 89.98 degrees, gamma = 110.46 degrees with two molecules in the crystallographic asymmetric unit. Form II has a noncrystallographic twofold axis, Intensity data to 3.1 Angstrom resolution for form I and to 2.2 Angstrom resolution for form II have been collected. (C) 1997 Academic Press.