In ragweed-sensitive and ragweed non-sensitive subjects the proportions of T lymphocytes bearing receptors for the Fc portion of IgG (T gamma) and IgM (T mu) were examined as obtained from the blood, and after treatment in vitro with ragweed antigen E or concanavalin-A. The proportion of T gamma and T mu cells, from the peripheral blood of ragweed-sensitive and ragweed non-sensitive persons, untreated in vitro, were not statistically different. However, when T cells from ragweed-sensitive subjects were exposed to ragweed antigen E, the T mu subpopulation was significantly increased (P less than 0.001) without change in the T gamma population. The reverse change occurred when cells of ragweed non-sensitive subjects were treated with antigen E; there was an increase in the T gamma subpopulation (P = 0.01) but no change in number of the T mu cells. Cells from both the sensitive and non-sensitive groups showed increase in number of T gamma and reduced numbers of T mu cells when incubated with concanavalin A. Since T gamma and T mu cells appear to have a regulatory function on B lymphocyte differentiation and antibody production, the pattern of responses of T gamma and T mu subpopulations in vitro to antigen E in ragweed-sensitive and ragweed non-sensitive subjects may reflect a difference in the cellular control of the immune response to ragweed antigen E.