CD43 is an integral cell membrane mucin appearing on hematopoietic cells in embryonic life probably on cells of vitellus bag, in embryonic and foetal liver and in foetal bone marrow. In adults CD43 occurs both on bone marrow hematopoietic sten cells as well as on peripheral mature white blood leukocytes with the exception of resting B lymphocytes. CD43 is also found on tissue macrophage, dendritic cells, smooth muscle cells, epithelium and endothelium. CD43 expression was detected on cells of leukaemias myeloid and lymphoid, lymphoma cells and metastases of solid neoplasms. Due to the elongated, twig-resembling shapes and numerous negatively charged rests of sialic acid CD43 can act in an anti-adhesive fashion e.g. disturbing ICAM-1 (CD54) ? LFA-1 (CD11A/CD18) interaction. Alternatively, in certain conditions CD43 can be pro-adhesive. So, e.g. adhesion hematopoietic progenitor cells to stromal bone marrow cells partly depends on CD34-CD43 cooperation. Similarly, according to the degree of cell maturity and the kind of cell line, CD43 can induce either activation or cell apoptosis. So, binding CD43 by specific monoclonal antibody induces activation and proliferation of T cells and also the oxidation burst of monocytes and neutrophils. On the other hand, CD43 induces apoptosis, especially in dividing progenitor hematopoietic stem cells.