m-Calpain constitutes the prototype of the superfamily of neutral calcium-activated cysteine proteinases. It is a heterodimer consisting of an 80 and a 30 kDa subunit. Recombinant full-length human m-calpain has been crystallized using macro-seeding techniques and vapour-diffusion methods. Two different monoclinic crystal forms (space group P2(1)) were obtained from a solution containing polyethylene glycol (M(W) = 10 000) as a precipitating agent. Complete data sets have been collected to 2.3 and 3.0 A resolution using cryo-cooling conditions and synchrotron radiation. The unit-cell parameters are a = 64.86, b = 133.97, c = 78.00 A, beta = 102.43 degrees and a = 51.80, b = 171.36, c = 64.66 A, beta = 94.78 degrees, respectively. The V(m) values indicate that there is one heterodimer in each asymmetric unit.