A highly metastatic hybrid line of human melanoma and tumorigenic, poorly metastatic hamster cells (BHK-21) was selected by pulmonary metastasis after injection of a fusion mixture into a Syrian hamster. The in vivo growth properties, karyotype, and plasminogen activator production of the line are reported. Monoclonal antibodies against the line indicated that it was expressing at least 8 human antigens although no melanoma-specific antigens were detected.