Disclosed herein are inhibitors, such as antibodies, and antigen bindingportions thereof, that selectively bind complexesof LTBP1-TGF.beta.1 and/or LTBP3-TGF.beta.1. The application also providesmethods of use of these inhibitors for, for example, inhibitingTGF.beta.1 activation, and treating subjects suffering from TGF.beta.1-relateddisorders, such as fibrotic conditions. Methods of selecting acontext- dependent or context-independent isoform-specific TGF.beta.1inhibitor for a subject in need thereof are also provided.
