We studied 28 lung carcinomas representing different histological types and three of their regional lymph node metastases immunohistochemically by using specific antibodies against two basement membrane proteins--the 7S domain of type IV collagen and the P1 fragment of laminin. One feature common to all peripherally growing tumors, regardless of the histologic type, was an intact basement membrane between the tumor and the unaffected lung tissue. At these locations, the basement membrane was organized into alveolar structures that did not differ from normal lung tissue. The fibrotic central areas of the tumors did not exhibit this phenomenon. Based on these findings, we believe that malignant tumors of the lung utilize preserved alveolar basement membranes for their local spread. This finding seems to represent a general property of all lung carcinomas, not only adenocarcinomas of the bronchiolo-alveolar type.