The effector function of activated CD4(+) T cells and secretion of cytokines are important in the establishment of productive immune responses and tolerance. We identified expression by CD4(+) T cells of Notch receptors and ligands and enhanced Notch signaling upon activation. Notch1 expression was up regulated and co-localized with CD4 upon T cell stimulation. Disruption of Notch signaling did not affect proliferation, but attenuated cytokine secretion following CD3 ligation in the absence of anti-CD28 antibody. Notch signaling was absolutely necessary for transcription of IL-10 by stimulated CD4(+) T cells. CD4(+) T cells transfected with constitutively active Notch1 failed to proliferate, but exhibited enhanced cytokine secretion upon stimulation. Our data indicates that Notch receptor signaling can influence both proliferative and. cytokine responses of CD4(+) T cells. In addition, the finding that Notch signaling is required for production of IL-10 may allude to a role in immune regulation.