The catalytic domain of human phosphodiesterase 3B has been cloned, expressed in Escherichia coli and purified in the presence of the PDE3 inhibitors IBMX (3-isobutylmethylxanthine) or MERCK1 by affinity chromatography. Initial screening of crystallization conditions for these complexes in the hanging-drop vapor-diffusion mode resulted in three different crystal forms, all characterized by quite large unit-cell parameters, elevated solvent content and poor diffraction quality. Subsequent optimization of these conditions led to crystals that diffract to 2.4 A and belong to space group C2, with unit-cell parameters a = 146.7, b = 121.5, c = 126.3 A, beta = 100.6 degrees. Rotation-function analysis indicates that the asymmetric unit contains four copies of the monomeric enzyme, corresponding to a solvent content of 64%. To solve the structure of the PDE3B catalytic domain, molecular replacement as well as multiple isomorphous replacement methods are currently being utilized.