A series of rat 13762NF mammary adenocarcinoma cell clones and subclones of various lung metastatic potentials were examined for their abilities to degrade rat lung subendothelial matrix and purified basement membrane type IV collagen. Metastatic potentials were simultaneously determined based on the ability to form “spontaneous” lung metastases after s.c. injection or “experimental” lung metastases after i.v. injection of cells. Microvessel endothelial cells isolated from rat lung were grown in vitro, and the subendothelial matrix containing type IV collagen was metabolically labeled with [3H]proline. When mammary adenocarcinoma cells were placed on the isolated subendothelial matrix, fragmentation and solubilization of [3H]proline-labeled components were observed; highly metastatic 13762NF cells solubilized the matrix at higher rates than did poorly metastatic cells. The 13762NF cells were assayed for type IV collagenolytic activity using [3H]proline-labeled type IV collagen purified from Engelbreth-Holm-Swarm tumor as a substrate. We found excellent correlation between the type IV collagenolytic activities of living cells and their “spontaneous” lung metastatic potentials (r = 0.993). The levels of type IV collagenolytic activity in the conditioned medium depended on the cell culture conditions. In the presence or absence of acid-treated fetal bovine serum, highly metastatic cells secreted higher amounts of type IV collagenolytic enzymes in active and latent forms than did poorly metastatic cells. Incubation of procollagen type IV with medium conditioned by highly metastatic 13762NF cells and treated with trypsin resulted in the production of several large fragments characteristic of type IV collagen. The results suggest that enzymatic degradation of basement membrane type IV collagen is important in lung metastasis of 13762NF mammary adenocarcinoma cells. © 1987, American Association for Cancer Research. All rights reserved.