A panel of 377 commercially available mAbs was submitted to the animal homologue section of the Eighth International Workshop on Human Leukocyte Differentiation Antigens (HLDA8, Adelaide, Australia) for cross-reactivity studies in a range of vertebrate species. Eight commercial suppliers participated by providing isotype controls and mAbs specific for a total of 144 CD antigens. In this study, we describe the results of flow cytometric testing of the reactivity of these mAbs with leukocyte populations isolated from blood, bronchoalveolar lavage, and ileal Peyer's patches of sheep. A total of 52 mAbs were identified as potentially reacting with sheep blood leukocytes in the first round of screening with blood leukocytes. In the second phase, reactivity of selected mAbs was further analyzed by repeating the screening with blood leukocytes at an independent facility. Screening of selected mAbs for reactivity with myeloid antigens was completed with alveolar macrophages and screening for reactivity with B cell antigens was completed with ileal Peyer's patch B cells. This screening identified mAbs that consistently reacted with both putative myeloid (CD10, CD22, CD23, CD27, CD29, CD32, CD49d, CD81, CD86, CD88, CD163, CD165) and B cell (CD10, CD22, CD23, CD27, CD29, CD32, CD49d, CD81, CD86, CD88, CD165) activation or differentiation antigens. Further studies will be required to determine if each mAb cross-reacts with an orthologous leukocyte antigen.