The acid-insoluble fraction of murine tumor basement membrane produced by the EHS sarcoma was extracted following reduction of disulfide bonds under non-denaturing conditions. Extracts prepared from tumors grown in either lathyritic or normal animals contain two procollagen-like chains as well as cross-linked oligomers of these chains. Extracted monomeric chains behave almost exactly like reduced, radiolabeled EHS collagen precursor chains on SDS-PAGE and SDS-agarose suggesting that they are very similar collagens. Since the collagen extracted after reduction represented a significant proportion of the total tumor collagen, it is likely that tumor basement membrane is assembled from precursor-size Type IV collagen molecules.