The low density lipoprotein receptor-related protein (LRP) is an endocytic receptor that is a member of the low density lipoprotein receptor family. We report that the LRP ligand, activated α₂-macroglobulin (α₂M*), induces robust calcium influx in cultured primary neurons, but not in nonneuronal LRP-containing cells in the same culture. The calcium influx is mediated through N-methyl-d-aspartate receptor channels, which explains the neuron specificity of the response. Microapplication of α₂M* leads to a localized response at the site of application that dissipates rapidly, suggesting that the calcium signal is temporally and spatially discrete. Calcium influx to α₂M* is blocked by the physiological LRP inhibitor, receptor-associated protein. Bivalent antibodies to the extracellular domain of LRP, but not Fab fragments of the same antibody, cause calcium influx, indicating that the response is specific to LRP and may require dimerization of the receptor. Thus, LRP is an endocytic receptor with a novel signaling role.