Various immunologic parameters were tested in a family of two patients with common variable immunodeficiency. Both patients had low serum immunoglobulin levels, low peripheral B cell and T4 subclass of lymphocytes, and reversed T4 T8 ratio. One of the patients had excessive suppressor activity. Two of the asymptomatic members of the family (the father and one brother) had also low T4 T8 ratio that was not associated with excessive suppressor activity. No linkage between the disease inheritance to HLA could be observed. A study of the T cell helper activity to an antigen, the response to which is regulated by an HLA-linked gene, suggested a defect in the immune response of the two patients and their asymptomatic brother with immunologic disorders. Treatment with cimetidine of the patient with excessive suppressor activity led to an improvement in his clinical state, reduction in suppressor activity, temporary effect on his proliferative response capacity to mitogens, and an increase in the antigen-specific helper activity.