Mice homozygous for a SCID mutation (SCID mice) are severely deficient in T and B lymphocytes. The absence of effector T and B cells has encouraged investigators to attempt engraftment of SCID mice with human fetal tissues, mature lymphocytes, hematopoietic progenitors and tumors. SCID mice can be reconstituted with human lymphocytes and are of interest for studying normal and abnormal lymphocyte development and function. SCID mice are also providing an in vivo model of infectious diseases. In addition, SCID mice readily support normal and pathologic human hematopoiesis differentiation and is useful for testing innovative hematological disease therapy. SCID mice with a fully functional human immune or hematopoietic system therefore seem to be extremely valuable for biomedical research.