Spontaneously occurring microscopic lung and lymph node metastases (in athymic mice) of a low metastatic human lung carcinoma cell line, UCP3, and its high metastatic variant, MV522, were isolated. Characterization of the variants included karyotypic and isoenzyme analyses; assessment of spontaneous metastatic capabilities in athymic mice, and monoclonal antibody analyses. The high metastatic variant LNT had barely detectable amounts of a glycoprotein molecule with apparent Mr 73 kd and 90 kd, which was present in the other cell lines. This molecule was detected in 20/24 primary human neoplasms but only in 3/18 metastatic neoplasms, suggesting a loss during the metastatic disease process.