A polymerase chain reaction method was carried out to address the possible presence of estrogen receptor (ER) mutations in murine transformed cell lines. The segment of ER cDNA coding the C and D domains was amplified and cloned into pUC 19. Mammary carcinoma cell line (SC-3) did not show any mutation in this segment. However, the sequence analysis of ER in the Leydig cell line (B-1 F) revealed a single base change at acidic Glu-279 (GAA) to basic Lys-279 (AAA) compared with murine uterus ER cDNA. The biological significance of this mutation is discussed.