Suppressor-cell activity of Concanavalin-A-stimulated lymphocytes was studied in allergic patients by inhibition of one-way mixed lymphocyte culture reactions before and after allergy immunotherapy. This activity was compared with twelve healthy controls. In preliminary experiments, six out of eight allergic patients had no detectable T suppressor activity. In the second prospective group, eight out of eleven patients had much reduced suppressor-cell activity before immunotherapy, and seven out of eleven patients had much reduced activity after immunotherapy. The data suggest that non-specific T suppressor-cell activity is reduced in allergic patients but immunotherapy does not restore such activity.