The activity of TGF-beta1 is regulated primarily extracellularly where the secreted latent form must be modified to expose the active molecule. Here we show that thrombospondin-1 is responsible for a significant proportion of the activation of TGF-beta1 in vivo. Histological abnormalities in young TGF-beta1 null and thrombospondin-1 null mice were strikingly similar in nine organ systems. Lung and pancreas pathologies similar to those observed in TGF-beta1 null animals could be induced in wild-type pups by systemic treatment with a peptide that blocked the activation of TGF-beta1 by thrombospondin-1. Although these organs produced little active TGF-beta1 in thrombospondin null mice, when pups were treated with a peptide derived from thrombospondin-1 that could activate TGF-beta1, active cytokine was detected in situ, and the lung and pancreatic abnormalities reverted toward wild type.