Objective To breed a new severe combined immunodefecient 615-SCID mouse with 615 genetic background. Methods The scid mutant gene of SCID mouse was transferred into 615 inbred strain by cross-backcross method. Results The new established 615-SCID mouse with brown hair like 615 mouse is homozygous for the scid mutant gene lacking functional T and B lymphocytes. The conspicuous histological features of the 615-SCID mouse was observed. The total leukocyte count of the peritoneal cavity of 615-SCID varied from 1500 to 4500/mm 3. The proportion of lymphocytes is below 30%. B(CD19) and T(CD3, CD4, CD8) lymphocytes are both similar to SCID. But the proportion of T cell is less than SCID mouse. ELISA-test found that the serum Ig is less than 0.02mg/ml. The thymus glands were very small, usually comprising one or two lobules and occasionally undetectable grossly. No lymphocytic cortex is evident in thymus. The spleen varied in size, smaller than SCID mouse. The splenic folliculi are virtually absent. And lymph nods have only cortex. Conclusion 615-SCID mouse expresses an obvious sensitivity to radiation, lacks detectable B and T cells.