Transforming growth factor (TGF) beta 1 mRNA is selectively stabilized during 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced differentiation of U937 promonocytes. In previous studies (R. Wager and R. Assoian, Mol. Cell. Biol., 10: 5983-5990, 1990), we showed that this phenotype results from the action of a RNase system that (a) recognizes the transcript selectively and (b) is inhibited upon exposure of cells to TPA. The studies reported here were designed to localize domains of TGF-beta 1 mRNA required for recognition by this TPA-regulated, transcript-selective RNase system. By examining the degradation of several truncated TGF-beta 1 in vitro transcripts with U937 cell extracts, we show that the coding domain is sufficient to allow selective degradation of the mRNA and that this process is enhanced by either the 5' or 3' untranslated regions. The 5' and 3' untranslated regions of TGF-beta 1 mRNA are also required for TPA-mediated inhibition of the transcript-selective RNase system. In contrast, an analysis of the half-lives of the 2.1- and 1.8-kilobase TGF-beta 1 mRNAs showed that the first 270 bases, unique to the larger TGF-beta 1 mRNA, minimally affect degradation of the transcript. Finally, a survey of several transcripts showed that gamma-actin mRNA levels are also controlled by the TPA-regulated RNase system. The regulated decay of TGF-beta 1 mRNA may reflect the behavior of a class of transcripts subject to similar posttranscriptional controls on overall gene expression.