For the First International Swine CD Workshop an effort was made to include commercially available anti-human CD monoclonal antibodies (mAb) that had been prescreened to determine whether they were cross-reactive with swine cell CD antigens. Thus before the actual workshop began, ten companies were contacted to determine whether they would participate in this effort. Four companies (Dako Corporation, Carpenteria, CA; T Cell Sciences, Cambridge, MA; Bioproducts for Science, Indianapolis, IN; PharMingen, San Diego, CA) provided a total of 38 mAb for tests of cross-reactions with swine cell antigens. Two companies (Coulter, Hialeah, FL and Becton Dickinson, San Jose, CA) declined to participate because of the low likelihood of positive cross-reactions. Several other companies (Behringewerke, Marburg, Germany; Boehringer Mannheim, Indianapolis, IN; Biosource International, Camarillo, CA; Applied Immune Sciences, Menlo Park, CA) were interested but either had problems in preparing the mAb in time for US importation or in providing the quantities necessary free to the workshop. The studies described below verify that few anti-human CD mAb exhibit consistent, predictable cross-reactions on pig cells; of the seven mAb that showed reactivity on swine cells only one mAb, Dako's anti-CD18, showed consistent reactivity with every pig tested and at the expected intensity level.