Synthetic peptides containing the arginine-glycine-aspartate (RGD) motif have been used extensively as inhibitors of integrin-ligand interactions in studies of cell adhesion, migration, growth and differentiation(1-3), because the RGD motif is an integrin-recognition motif found in many ligands. Here we report that RGD-containing peptides are able to directly induce apoptosis without any requirement for integrin-mediated cell clustering or signals. We show that RGD-containing peptides enter cells and directly induce autoprocessing and enzymatic activity of pro-caspase-3, a pro-apoptotic protein. Using the breast carcinoma cell line MCF-7, which has a functional deletion of the caspase-3 gene, we confirm that caspase-3 is required for RGD-mediated cell death, In addition to an RGD motif, pro-caspase-3 also contains a potential RGD-binding motif, aspartate-aspartate-methionine (DDM)(4), near the site of processing to produce the p12 and p17 subunits(5). On the basis of the ability of RGD-DDX interactions to trigger integrin activation(6), we suggest that RGD peptides induce apoptosis by triggering conformational changes that promote pro-caspase-3 autoprocessing and activation. These findings provide an alternative molecular explanation for the potent pro-apoptotic properties of RGD peptides in models of angiogenesis, inflammation and cancer metastasis(7-9).