A case of progressive vaccinia associated with a profound deficiency of cellular immunity and a defect in inosine phosphorylase is described. A striking contrast was observed between humoral immunity, which showed little if any impairment, and a severe cellular defect affecting both markers and functions of T lymphocytes. The child died despite treatment with methosazone, levamisole, transfer factor, irradiated blood transfusions, and a thymus graft. An adequate serum level of antibody to vaccinia virus was obtained by transfer of specific immunoglobulins, but this failed to stop the progression of the disease. This observation suggests that host defense against vaccinia infection is mainly mediated by cellular immunity.