THE protein elastin has been generally regarded as an inert substance on the basis of its solubility properties1 and tracer studies2 employing carbon-14. That elastic tissue demonstrates dynamic, functional properties other than that of elasticity has been suggested in part by studies on arteriosclerotics. The breakdown of the arterial elastic lamellae is generally reported to be the initial event in the pathological changes of the arteries3. That the elastic lamellae of arteries act as selective filters to tissue fluid components has been proposed4. The present report demonstrates that elastin, as it exists in situ in elastic tissue, is composed of metabolically active components.