Stimulation of T cells via the antigen and costimulatory receptors leads to the organization of a supramolecular activation cluster called the immune synapse. We report that loss of the molecular adaptor Cbl-b in T cells frees antigen receptor–triggered receptor clustering, lipid raft aggregation, and sustained tyrosine phosphorylation from the requirement for CD28 costimulation. Introduction of the cbl-b mutation into a vav1^−/− background relieved the functional defects of vav1^−/− T cells and caused spontaneous autoimmunity. Wiscott Aldrich Syndrome protein (WASP) was found to be essential for deregulated proliferation and membrane receptor reorganization of cbl-b mutant T cells. Antigen receptor–triggered Ca²⁺ mobilization, cytokine production, and receptor clustering can be genetically uncoupled in cbl-b mutant T cells. Thus, Cbl-b functions as a negative regulator of receptor clustering and raft aggregation in T cells.