Abstract The feasibility for microplate-based screening of inhibitors of isozymes of cyclic nucleotide phosphodiesterase (PDE) was tested via the coupled action of a phosphatase on adenosine-5'-monophosphate and an improved malachite green assay of phosphate. Human full-length PDE4B2 and truncated mutant (152-528aa) were expressed in Escherichia coli via fusion to SUMO, which after purification through Ni-NTA column exhibited specific activities >0.017 U mg(-1). In the presence of proteins <30 mg L(-1), absorbance for 10 µΜ phosphate was measurable; a PDE isozyme of specific activity over 0.008 U mg(-1) after reaction for 20 min thus suited for microplate-based screening of inhibitors. By using Biotek ELX 800 microplate reader, affinities of two forms of PEDE4B2 for cAMP, rolipram and papaverine varied over three magnitudes and were consistent with those by routine assay, respectively. Hence, the proposed method was promising for high-throughput-screening of inhibitors of phosphate-releasing enzymes bearing specific activities over 0.008 U mg(-1).