The catalytic subunit of the serine/threonine specific protein phosphatase 1 from human (molecular mass 37 KDa) has been co-crystallized in complex with the cyanobacterial toxin microcystin LR (molecular mass 1 kDa). The crystals diffract to a resolution of 2.8 A when exposed to synchrotron radiation and belong to space group P2(1)2(1)2 with a = 109.5 A, b = 90.6 A, c = 38.7 A. There is one molecule of protein phosphatase 1 per asymmetric unit. The crystal form is suitable for the determination of the atomic structure of protein phosphatase 1.