Three-hundred and eight monoclonal antibodies detecting surface determinants on ruminant cells were tested on canine leukocytes by flow cytometry. Thirty mabs showed crossreactivity: 25 of them reacted uniformly with cells of three unrelated dogs, five displayed a polymorphic reaction pattern. Thus, in addition to mabs with yet unknown specificity, antibodies to ruminant WC9, B-cell antigens and possible CD29, CD49d cross-react with canine cells, and probably react with the homologous canine antigen.