ABSTRACT. In various human tumors, a metal binding protein, metallothionein (MT) is reported to play an important role in carcinogenesis. In the present preliminary study, MT expression and tumor growth were investigated in transplantable pregnancy-independent mammary tumors (TPIMT) derived from pregnancy-independent mammary tumors (PIMT) in GR/A mice, in order to study the possible role of MT in mammary carcinogenesis. TPIMT as well as PIMT showed MT expression in tumor cells in all of the successive transplantations. A negative correlation was observed between MT expression in transplanted tumor tissues and their growth in the hosts (r = -0 .53, p<0.05). The present study indicates that MT is a useful marker of tumor progression in TPIMT. KEY WORDS: metallothionein, progression, transplantable pregnancy-independent mouse mammary tumor.