The present invention is directed to novel chimeric co-stimulatory receptor proteins and DNA sequences encoding these proteins. The chimeric receptors comprise at least three domains in a single chain molecule: an extracellular ligand binding domain, a transmembrane domain and a cytoplasmic co-stimulatory effector function signaling domain that acts synergistically with an effector function signal in the host cell. Novel hybrid co-stimulatory receptor proteins include a second cytoplasmic effector function signaling domain. The invention further relates to expression cassettes containing the nucleic acids encoding the novel chimeric receptors, to host cells expressing the novel chimeric receptors and to methods of using the receptors to co-stimulate effector functions in the cells and for using cells expressing the receptors for treatment of cancer, disease and viral infections.