The role that soluble binding proteins might play in regulating transforming growth factor-beta 1 (TGF-beta 1)-induced growth of smooth muscle cells (SMC) is unknown. alpha 2-Macroglobulin (alpha 2M) is the major plasma binding protein for TGF-beta. Reaction of alpha 2M with methylamine (alpha 2M-MA) forms "activated" alpha 2M which binds TGF-beta and specific cell surface receptors. The objectives of these studies were to determine whether native alpha 2M or alpha 2M-MA influences growth responses of cultured rat aortic SMC to TGF-beta 1. Results demonstrated that native alpha 2M was not mitogenic. Treatment with alpha 2M-MA or TGF-beta 1 stimulated a 3- or 3.5-fold increase in [3H]thymidine incorporation, respectively. Cotreatment with TGF-beta 1 and alpha 2M-MA resulted in a 70-fold increase in [3H]thymidine incorporation. SMC bound alpha 2M-MA in a specific and saturable manner and expressed alpha 2M receptor/low density lipoprotein receptor-related protein (LRP). A modified form of alpha 2M-MA (alpha 2M-MA-cis-dichlorodiammine platinum), which bound TGF-beta 1 but did not bind alpha 2M receptors, failed to enhance TGF-beta 1-induced growth. In summary, results demonstrated that alpha 2M-MA enhanced TGF-beta 1-induced growth responses and that this effect was dependent on alpha 2M-MA binding to alpha 2M receptor/LRP.