We describe a 2 year old girl with nucleoside phosphorylase (PNP) deficiency, who had low blood T cell numbers and T lymphocyte blastogenic response to mitogens, hypergammaglobulinaemia, high titres of antibodies to many common antigens, various autoantibodies, a monoclonal IgM-kappa protein, an increased frequency of mature Ig containing blood B cells and a high production of Ig in vitro in unstimulated cultures. E rosetting cells showed faint or no immunofluorescence staining with monoclonal antibodies directed against T cell membrane antigens. In vitro Ig production in response to pokeweed mitogen was defective, and no T cell helper or suppressor activity was observed. It is suggested that the immunoregulatory deficiency might have caused the B cell hyperactivity.