Metastatic disease is one of the major causes of death from cancer in human beings. Several enzyme systems have been implicated in the metastatic process, but the metalloproteinases (MPs) appear to be the major group involved in most instances of neoplastic invasion. Increased MP activity has been correlated with the metastatic potential of many cancers, including breast cancer. MPs also play a role in tumor angiogenesis. Tetracyclines are antimicrobial agents that can suppress MP activity in a variety of tissues, including gingiva, bone, and cartilage. Several reports have indicated that tetracyclines can suppress tumor MPs as well. A synthetic tetracycline, doxycycline, inhibits migration of human MDA-MB-435 breast adenocarcinoma cells through a reconstituted basement membrane (Matrigel), an assay used as an in vitro surrogate for the in vivo process of tumor invasion through basement membranes. Additionally, doxycycline diminishes the proliferation of this breast cancer cell line and also decreases its gelatinolytic activity, as determined by gel zymography.