Fibroblast differentiation to the myofibroblast phenotype is associated with α–smooth-muscle actin (α-SMA) expression and regulated by cytokines. Among these, transforming growth factor (TGF)-β₁ and interleukin (IL)-1β can stimulate and inhibit myofibroblast differentiation, respectively. IL-1β inhibits α-SMA expression by inducing apoptosis selectively in myofibroblasts via induction of nitric oxide synthase (inducible nitric oxide synthase [iNOS]). Because TGF-β is known to inhibit iNOS expression, this study was undertaken to see if this cytokine can protect against IL-1β–induced myofibroblast apoptosis. Rat lung fibroblasts were treated with IL-1β and/or TGF-β₁ and examined for expression of α-SMA, iNOS, and the apoptotic regulatory proteins bax and bcl-2. The results show that TGF-β₁ caused a virtually complete suppression of IL-1β–induced iNOS expression while preventing the decline in α-SMA expression or the myofibroblast subpopulation. TGF-β₁ treatment also completely suppressed the IL-1β–induced apoptosis in myofibroblasts. IL-1β–induced apoptosis was associated with a significant decline in expression of the antiapoptotic protein bcl-2, which was prevented by concomitant TGF-β₁ treatment. The level of the proapoptotic protein bax, however, was not significantly altered by either cytokine. These data suggest that TGF-β₁ inhibits IL-1β–induced apoptosis in myofibroblasts by at least two mechanisms, namely, the suppression of iNOS expression and the prevention of a decline in bcl-2 expression. Thus, TGF-β₁ may be additionally important in fibrosis by virtue of this novel ability to promote myofibroblast survival by preventing the myofibroblast from undergoing apoptosis.