Two monoclonal antibodies (LAM-A and LAM-B) specific for laminin from normal and neoplastic parietal yolk sac (PYS) cells were produced in rats immunized with a mouse yolk sac carcinoma cell line. Both antibodies immunoprecipitated the 400,000- and 200,000-Da chains of laminin and reacted with purified PYS laminin in ELISA. LAM-A reacted with mouse and rat PYS laminin, whereas LAM-B reacted only with mouse PYS laminin. Formaldehyde- and methanol-fixed adult and fetal somatic tissues were immunohistochemically unreactive with either of the two antibodies. In acetone-fixed tissue sections, both antibodies reacted weakly with some medullary tubules of the kidney, the follicular basement membrane of the ovaries, and the seminiferous tubules. The antibodies appear to react with the polypeptide determinants residing on the terminal portion of the long arm of laminin. It is concluded that laminin derived from normal or malignant PYS cells has distinct antigenic sites that are immunochemically not apparent in most other basement membranes.