Elastic fibers are progressively lysed during maturation and aging and in an accelerated fashion in several aging diseases such as diabetes, arteriosclerosis, emphysema and several skin diseases. Several enzymes (elastase-type proteases) were isolated in recent years in our laboratory which appear to be involved in these processes. A cell membrane bound serine protease was isolated from arterial smooth muscle cells and was shown to increase with in vitro aging of the cells. A metallo-protease was isolated from skin fibroblasts and was shown to be capable of attacking the constituents of elastic fibers, mainly the microfibrillar glycoproteins and also the desmosine cross linked elastin in vivo. This partially purified fibroblast enzyme was shown to attack these elastic fibers when injected into the dermis. A new selective staining procedure was used to visualise and quantitate, by computerized image analysis, the skin elastic fibers in normal and pathological human or animal skin biopsies. This method, combined with the injection of elastase in rabbit skins, alone or together with inhibitors, enables the ex vivo/in vivo study of elastase action (and of its inhibition).