Spontaneous Leydig cell tumors in inbred laboratory mice.

Six Leydig cell tumors were identified in 6,500 male mice necropsied over a period of 8.5 years. All tumors developed spontaneously in BALB/cJ and BALB/cByJ inbred mice. When submitted for necropsy, affected mice were 8 to 9 months old. The incidence was calculated to be 5.5 per 100,000 males retained as breeders. Tumors were unilateral, with no predilection for right or left testis, Histologically all Leydig cell tumors were well differentiated and of the solid, diffuse type.