Thirty-seven subpanels of monoclonal antibodies (mAbs) included within the Vth International Workshop on Human Leucocyte Differentiation Antigens (Vth Workshop) were assayed for reactivity with bovine peripheral blood leucocytes. Sixty-five of the 772 mAbs (8.4%) stained bovine cells. mAbs from each of the 27 different CD groups that contained a mAb reacting with cattle were further investigated to compare the cellular expression of the antigen in cattle with that reported for the different CD antigens in humans. Two-colour immunofluorescence staining of the Vth Workshop mAbs against characterized bovine leucocyte subpopulation markers that identified monocytes, B cells, CD4, CD8 and WC1 +T cells were used for these analyses. Eighteen of the mAbs to different human CD antigens (CD11a, CD14, CD18, CD21, CD27, CD29, CD49a, CD49b, CD49d, CD49e, CD51, CD61, CD62L, CD62P, CD63, CDw78, CD98, CD100) stained bovine antigens with an almost identical cellular distribution to that reported in humans. This implies that these mAb react with the homologous cattle molecules. Nine mAbs (CD35, CD37, CD49c, CD50, CD54, CD66, CD81, CD88, CD102) stained bovine cells but the cellular distribution of the bovine antigen was different to that reported in humans implying either a different cellular distribution for these antigens in cattle or a reaction with a different molecule. The investigation has allowed the identification of several bovine homologues of human CD antigens that have not been previously defined in cattle and the cross-reacting mAbs will be valuable reagents for future investigations of bovine immunology.