The expression of 14-3-3 proteins is dysregulated in various types of cancer. This study was undertaken to investigate the effects of 14-3-3 ${zeta}$ and 14-3-3 ${sigma}$ on cell growth inhibition mediated by transforming growth factor-beta 1 (TGF-${eta}$1). Mouse mammary epithelial cells (Eph4) that are transformed with oncogenic c-H-Ras (EpRas) and no longer sensitive to TGF-${eta}$1-mediated growth inhibition displayed increased expression of 14-3-3 ${zeta}$ and decreased expression of 14-3-3 ${sigma}$ compared with parental Eph4 cells. Using small interfering RNA-mediated knockdown and overexpression of 14-3-3 ${sigma}$ or 14-3-3 ${zeta}$, we showed that 14-3-3 ${sigma}$ is required for TGF-${eta}$1-mediated growth inhibition whereas 14-3-3 ${zeta}$ negatively modulates this growth inhibitory response. Notably, overexpression of 14-3-3 ${zeta}$ increased the level of Smad3 protein that is phosphorylated at linker regions and cannot mediate the TGF-${eta}$1 growth inhibitory response. Consistent with this finding, mutation of the 14-3-3 ${zeta}$ phosphorylation sites in Smad3 markedly reduced the 14-3-3 ${zeta}$-mediated inhibition of TGF-${eta}$1-induced p15 promoter-reporter activity and cell cycle arrest, suggesting that these residues are critical targets of 14-3-3 ${zeta}$ in the suppression of TGF-${eta}$1-mediated growth. Taken together, our findings indicate that dysregulation of 14-3-3 ${sigma}$ or 14-3-3 ${zeta}$ contributes to TGF-${eta}$1 resistance in cancer cells.