In mammals, AMP-activated protein kinase (AMPK) is a heterotrimeric protein composed of a catalytic serine/threonine kinase subunit (alpha) and two regulatory subunits (beta and gamma). The gamma subunit senses the intracellular energy status by competitively binding AMP and ATP and is thought to be responsible for allosteric regulation of the whole complex. This work describes the purification and preliminary crystallographic analysis of protein MJ1225 from Methanocaldococcus jannaschii, an archaeal homologue of gamma-AMPK. The purified protein was crystallized using the hanging-drop vapour-diffusion method. Diffraction data for MJ1225 were collected to 2.3 A resolution using synchrotron radiation. The crystals belonged to space group H32, with unit-cell parameters a = b = 108.95, c = 148.08 A, alpha = beta = 90.00, gamma = 120.00 degrees . Preliminary analysis of the X-ray data indicated that there was one molecule per asymmetric unit.