A congenic C57BL/6JJcl-Tcrb^tm1MomTrp53^tm1 (Tcrb ^-/-: Trp53 ^-/-) mouse lacking T-cell receptor beta chain (TCRβ) and transformation related protein 53 (p53) has been established at the N8th generation of backcrossing male Tcrb ^-/- :Trp53 ^-/- mice, which had been obtained by mating a Tcrb ^-/- mouse with a Trp53 ^-/- mouse, with female C57BL/6JJcl mice. In the mice deficient for the both genes, occurrence of tumor masses was observed mostly in the cecum with high frequency as examined at 3 months of age. The majority of the masses had histologic features of hyperplasia or dysplasia while occasional lesions were noted to be adenocarcinomas invading the submucosa (invasive adenocarcinoma). As examined at 4 months of age and thereafter, all mice had 4-5 colorectal tumors per animal, the lesions being located mainly in the cecum and, histopathologically, all the obvious neoplastic growths in the regions examined were invasive adenocarcinomas. The Tcrb and Trp53 genes deficient mouse strain which develops spontaneous colorectal carcinoma with fairly high frequency at early age would be useful as an animal model for colorectal cancer.