Four monoclonal antibodies directed against ovine/bovine leucocyte antigens (Co.44B8, Co.11F10, Co.33B3 and Co.26F4) were produced and all recognized an antigen with an apparent molecular weight of 105 kDa under reducing conditions. This molecule was considered to be the ovine/bovine analogue of human, rat and mouse CD43 or leucosialin. This conclusion was based on (a) the cellular distribution of the antigen which was similar to that described in other species, (b) a decrease of apparent M(r) (150 kDa) after treatment with neuraminidase, and (c) production of cell aggregation by the four monoclonal antibodies and the influence of temperature and metabolic inhibitors on this phenomenon. Co.44B8, Co.11F10, Co.33B3 and Co.26F4 all recognized and immunoprecipitated the molecule after neuraminidase treatment of the cells. The data indicate an important function of CD43 as a mediator of cell aggregation for ruminant cells.