Transforming growth factor-betas (TGF betas) are multifunctional peptides that regulate growth and differentiation in a variety of cells. The goals of this study were to compare expression of the TGF beta isoforms in normal myometrium and benign leiomyoma tumors of the uterus and to examine the effects of TGF betas on cell proliferation and collagen production by these cells in vitro. Myometrium and leiomyoma tissues were obtained from patients undergoing elective hysterectomies. Tissues were processed for ribonucleic acid (RNA) and were also established as primary cell cultures. Northern blot analysis showed that the levels of TGF beta 1 messenger RNAs (mRNAs) were similar between leiomyoma and myometrium, whereas leiomyoma showed 5-fold higher levels of expression of TGF beta 3 mRNA than autologous myometrium. Expression of TGF beta 3 protein detected by immunohistochemistry was much more intense in leiomyoma tissues than in corresponding myometrium. Levels of both TGF beta 1 and TGF beta 3 increased with increasing cell density for leiomyoma and myometrium smooth muscle cells cultured in vitro. Effects of TGF beta 1 and TGF beta 3 on cell proliferation were assessed by measuring changes in DNA synthesis with the tritiated thymidine incorporation assay. The doses of TGF betas tested were 0, 0.1, 1.0, and 10.0 ng/mL. All three doses of TGF beta 1 and TGF beta 3 inhibited DNA synthesis in myometrium smooth muscle cells by 31--54%. Concomitant treatment with an immunoneutralizing antibody to TGF beta 1--3 reversed this inhibitory effect. In contrast, TGF beta 1 had no effect on leiomyoma smooth muscle cells, whereas TGF beta 3 increased DNA synthesis by leiomyoma cells. Combined treatment with the immunoneutralizing antibody prevented this increase. Treatment of leiomyoma and myometrial cells with the TGF beta immunoneutralizing antibody for 24 h caused a 45--60% reduction in collagen type I and type III mRNA levels, suggesting that endogenous TGF betas are important for collagen production. These results support the hypothesis that alterations in the TGF beta system produce loss of sensitivity to the antiproliferative effects of TGF beta, and increased expression of TGF beta 3 may contribute to the growth of these tumors.