The present invention is novel derivatives of pyrrolo[3,2-d]pyrimidines and pharmaceutical compositions and methods of use therefor. The derivatives are inhibitors of purine nucleoside phosphorylase selectively cytotoxic to T-cells but not to B-cells in the presence of 2'-deoxyguanosine and, therefore, are for use in the treatment of autoimmune diseases, gout, psoriasis or rejection of transplantation.