A new human cell line MRT-1 was established from a metastatic lymph node of renal cell carcinoma of a 47 years old Japanese male. The cultured MRT-1 cells exhibit an epithelial appearance and contain lipid vacuoles in their cytoplasm in vitro. The modal chromosome number was 70. Doubling time and plating efficiency were 32.5 h and 5%, respectively (at the 47th passage). Injection of 1X10(6) MRT-1 cells beneath the renal capsule of nude mice resulted in tumor formation resembling the original tumor. Serine proteinase inhibitors produced by the cell line were analyzed. The cultured cells produced alpha 1-antitrypsin (alpha 1-AT) and plasminogen activator inhibitor-1 (PAl-1) into the conditioned medium. Most of the MRT-1 derived alpha 1-AT had lost the affinity to Concanavalin A when compared to the normal plasma alpha 1-AT.