Platelets may have a role in the development of animal tumor metastases. Ultrastructural studies in vivo have shown arrested tumor emboli surrounded by platelets. Several tumor cell lines induce thrombocytopenia in vivo. Certain tumor cells aggregate platelets in vitro. Correlations exist between the ability of some tumor cells to aggregate platelets in vitro and their metastatic potential in vivo. Antiplatelet agents have impaired or altered the spread of certain tumor metastases. It is suggested that platelets have a role in the sequestration, adherence, and penetration of tumor cells through the blood vessel endothelial cell barrier, thus preventing their rapid clearance from the circulation and allowing extravascular formation of nests of cells. Antiplatelet agents, particularly prostaglandins, may prove useful in preventing experimental animal metastases when administered before the inoculation of tumor cells. Their potential in human malignancy, where the patient presents with an established tumor, remains to be established.