A 28-year-old man with immunodeficiency with hyper IgM was studied. His serum immunoglobulins were characterized by the absence of IgA and low level of IgG associated with high level of IgM. The in vitro pokeweed mitogen (PWM)-induced immunoglobulin synthesis by his peripheral blood lymphocytes was depressed completely for IgA and moderately for IgG although normal numbers and proportions of IgA- and IgG-bearing lymphocytes (on the surface) were demonstrated in the peripheral blood. His T cells could not help IgA production by either normal or his own B cells, whereas they were more efficient helpers for IgM production by his own B cells than were normal T cells. In addition, his B cells produced no IgA and less IgG than normal B cells when co-cultured with normal T cells. This suggests that the failure of IgA specific T helper activity and the maturation arrest of B cells at the stage of the switchover from IgM- to IgG- and/or IgA-producing cells may be the major cause for the hypogammaglobulinaemia in this patient. It is uncertain whether the maturation arrest of B cells is secondary to the T cell defect in helper function for IgA production.