The collagens of fibrous atherosclerotic lesions of human aortae obtained at post mortem examination were compared with those of normal intima-media preparations. Assessed quantitatively, pepsin-solubilized types IV, V and VI collagens decreased in relation to types I and III in preparations from lesions as compared to values for controls. The type V collagen in both tissues were composed of alpha 1 (V) and alpha 2 (V) chains in a 2:1 ratio. A novel ("V") collagen polypeptide identical in size to the alpha 1 (V) chain was identified in association with the interstitial collagen fraction in both tissue types. This chain had unique solubility characteristics and cyanogen bromide peptide composition. The exact relation of this polypeptide to the other collagens is not known, but it is possible that it accounts for the reported fluctuations in type V chains in aortic tissues.