Abstract: Severe combined immunodeficiency (SCID) mice, deficient for both T and B lymphocytes, had been bred successfully in laminar air flow units. The life spans of most SCID mice were over 12 months, the average litter size 5.5, the reproductive and weaning rate were comparable to those of BALB/c mice. The results of microbiological examination showed that the animals were free from the specific pathogens. It was also revealed that the thymus and spleen were significantly smaller than those of BALB/c mice, circulating lymphocytes comprised only a small fraction of the total leukocytes in the peripheral blood (11%-26%)» while neutrophils amounted to 74%—87%. Three kinds of human tumor tissues (lung carcinoma, liver cell carcinoma and mucinous carcinoma of colon) xenografted in SCID mice grew much more faster than those in BALB/c nude mice.