Mitogen- and antigen-induced lymphocyte transformation was determined in peripheral blood lymphocytes from tobacco smokers and nonsmokers. Each group was evaluated by race, sex, and age subgroups. Male smokers as a group had depressed responses to phytohemagglutinin (PHA), concanavalin A, and pokeweed mitogen. The effect was most pronounced in all subjects on PHA stimulation. There was no apparent relationship to age or race. Antigenic stimulation of lymphocytes with mumps skin test antigen was not affected in any of the groups studied.