The suppressor cell activity of peripheral blood lymphocytes was investigated in wheezy children by determining the ability of concanavalin A induced suppressor cells to inhibit a mitogen provoked proliferative response. The activity was compared with that of eighteen healthy controls. Often infants aged from 5 months to 4½ years, with recurrent wheezy bronchitis, three had much reduced suppressor cell activity (2 s.d. or more below that of controls). Of twelve asthmatic children aged from 4½ to 12 years, seven had much reduced suppressor cell activity. The remaining five asthmatic children with normal suppressor cell activity had all received hyposensitization therapy. We speculate that immune hyposensitization therapy might act by stimulating lymphocyte suppressor activity.