Mice homozygous for the severe combined immune deficiency spontaneous mutation (Prkdcscid, commonly referred to as scid) are characterized by an absence of functional T cells and B cells, lymphopenia, hypogammaglobulinemia, and a normal hematopoietic microenvironment. Normal antigen-presenting cell, myeloid and NK cell functions are strain dependent. scid mice carry a DNA repair defect and a defect in the rearrangement of genes that code for antigen-specific receptors on lymphocytes. Most homozygotes have no detectable IgM, IgG1, IgG2a, IgG2b, IgG3, or IgA. Thymus, lymph nodes, and splenic follicles are virtually devoid of lymphocytes. scid mice accept allogeneic and xenogeneic grafts making them an ideal model for cell transfer experiments. Some scid mice will spontaneously develop partial immune reactivity. scid mice that have serum Ig levels greater than 1 ug/ml are considered "leaky." scid leakiness is highly strain dependent, increases with age, and is higher in mice housed under non SPF conditions. In general, scid leakiness is high on the C57BL/6J and BALB/cBy genetic backgrounds, low on the C3H/HeJ background, and even lower on the NOD/LtSz background. Hypogonadal mice (Ghrhhpg) mice are deficient in gonadotropins and gonodal steroids leading to an overall underdevelopment of the reproductive tract. Male mice have a small penis and scrotum and a short ano-genital distance. All male reproductive organs are present but immature. The testes are small and undescended. Spermatogenesis is arrested, usually by the diplotene stage. The vagina of female mice does not open fully, the uterus is thread-like, and the ovaries very small. Both sexes are sterile although fertility can be restored with hormone replacement. Ghrhhpg Prkdcscid double mutant mice are useful for determining endocrine dependence of grafted cells/tissues.