Collagenous components were extracted from bovine aorta by pepsin digestion. Differential salt precipitations separated the interstitial from the basement membrane (BM) collagens, and the latter were subsequently separated into three distinct types. Ion exchange chromatography, SDS-slab gel electrophoresis, cyanogen bromide and protease V8 peptide mapping, and amino acid analysis were used to characterize the component chains within each of these types. The major BM-class contained three distinct chains which were identical to the alpha 1(V), alpha 2(V) and alpha 3(V) chains of type V collagen from normal human placenta. The stoichiometry of the chains suggests a [alpha 1(V)]2 alpha 2(V)-helical organization, but the role of the alpha 3(V) chain in the overall structural organization of collagen V remains unknown. The second BM-class contained a heterogeneous group of molecules ranging in size from 40 000 to 140 000 daltons. Two predominant chains within this group were characterized as the alpha 1(IV) and alpha 2(IV) chains of type IV collagen. The last class of BM collagens consisted primarily of high molecular weight components; upon reduction these gave rise to two low molecular weight collagenous species (40 K and 45 K) characteristic of type VI, low molecular weight or 'linker' collagens. The functional roles of the isolated BM collagens, either individually or collectively, has not been ascertained to date.