The maturation of haematopoietic cells consists of their production (haematopoiesis) and their subsequent release into the marrow sinuses (haematoegression). The process by which the cells gain access to the circulation is selective in several regards, only mature cells at the end of the production line egress from the marrow and upon stimulation, only leucocytes leave the extracellular compartment of blood formation. Very little is known as to the regulatory processes which govern the egress of these cells in healthy individuals and their malignant counterparts in patients with leukaemias and lymphomas. Degradation of injured tissue is frequently involved in inflammation, and it has been proposed that proteinases released by macrophages and granulocytes might contribute by degrading protein components of extracellular matrices in a fashion proposed for tumor cells. Two major types of endopeptidases are believed to be involved in the migration of the cells through the basement membrane and the ensuing destruction of connective tissues : the matrix metalloproteinases (MMPs) and the plasminogen activators (PAs). Theses enzymes encompass three main areas in haematology : 1) leucocyte migration ; 2) tissue damage leading to extracellular matrix modification initiated by leucocytes during inflammation ; 3) progression of leukaemias and lymphomas. Moreover, MMPs, PAs, their receptors and their inhibitors have also been described as being implicated in chemotactism, mitogenesis and apoptosis.