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Nordic Immunohistochemical Quality Control (NordiQC) performs proficiency testing for about 600 pathology laboratories in more than 50 countries. All general results are published on www.nordiqc.org. Over-all, about 30% of the staining results on circulated slides from tissue micro arrays have by an expert group been assessed as insufficient for diagnostic use. This paper describes the results from the two latest NordiQC runs for CD31. Out of 476 stains submitted, 30.5% were considered insufficient, mostly due to too weak or false negative staining reactions. The best results were obtained by use of mouse monoclonal antibody JC70A with an optimized protocol based on efficient heat induced epitope retrieval and a three-step polymer/multimer conjugate as visualization system. The mouse monoclonal antibody 1A10 gave unsatisfactory results in almost all cases.