Leukosialin or CD43 is a heavily O-glycosylated transmembrane protein expressed on all cells of the haematopoietic cell lineage with the exception of red blood cells and mature B cells. This antigen has been identified in human, mouse and rat with monoclonal antibodies. Although orthologues of many human and rodent leucocyte cell surface antigens have been described in recent years, CD43, despite its abundance on human and rodent cells, remained uncharacterized in other vertebrate species. The comparison of CD43 amino acid sequences from human, mouse and rat indicated a high level of homology in the cytoplasmic domain. A serum, (p.aCD43cp) raised against the recombinant cytoplasmic tail of the human CD43, was shown not only to recognize human CD43, but it bound to putative CD43 orthologues in many mammalian species. CD43 was found to be expressed in the same leucocyte subpopulations and circumstantial evidence suggested that CD43 is also regulated similarly during leucocyte ontogeny in all species investigated. As CD43+ cells were readily observed in fixed tissues, the p.aCD43cp serum may be used as a reliable reagent for the verification of the haematopoietic origin of infiltrations and, used together with other reagents, for the serological characterization of normal and pathological lymphoid tissues and lymphoid infiltrations in experimental work and in animal disease.