It has been shown in recent studies that a group of histidine-rich proteins play a significant role--as interfilamentous matrix proteins--in keratin formation in the epidermis of man and other species. These proteins have been termed filaggrins in reference to their ability to aggregate in vitro reconstituted keratin filaments to macrofibrils with a striking similarity to the ultrastructural keratin pattern. Filaggrins derive from high molecular weight precursor proteins--profilaggrins--which are synthesized in the keratohyaline granules and break down rapidly into filaggrins when the granular cells undergo transition into corneocytes. Further metabolic breakdown of filaggrins leads to the formation of other compounds which subserve physiological functions: e.g. urocanic acid, which is possibly a natural sunscreen, and free amino acids, which are thought to contribute to the hygroscopic character of the horny layer and to its barrier function. Alterations of filaggrin metabolism have been recognized in a number of pathological states of the skin and may be responsible for disturbed epidermal differentiation.