To investigate the function of the β_C and β_E subunits of activin, we overexpressed these subunits in AML12 cells, a normal hepatocyte cell line, using adenovirus vector. Overexpression of the β_C subunit increased [³H]thymidine incorporation and the cell number. In contrast, both [³H]thymidine incorporation and the cell number were reduced in the β_E overexpressing cells. When AML cells overexpressing the β_E subunit were cultured in medium containing 1% serum for 48 h, many of the cells died by apoptosis, whereas cells overexpressing the β_C subunit or β-galactosidase survived in the same condition. To examine dimer formation, the β_C and β_E subunits were expressed in AML12 cells. In these cells, the β_C homodimer, the β_E homodimer and the β_C-β_E heterodimer were detected. When the expression level of the β_E subunit was increased, formation of the β_E homodimer was increased, while formation of the β_C-β_E heterodimer was slightly reduced. Overexpression of the β_E subunit did not significantly affect the formation of the β_C homodimer. These results indicate that the β_C and β_E subunits form homo- and heterodimers, and that the functions of the two subunits are quite different.