The role of the NZB/NZW F₁ (B/W) thymus was studied with respect to tolerance to ultracentrifuged bovine γ globulin (BGG) and poly I· poly C, and spontaneous anti‐DNA antibody formation. Thymectomy alone had little effect on the induction or escape from tolerance. An adult B/W thymus could not prevent toleranceinduction in young B/W mice. However, a young B/W thymus could facilitate the induction of partial tolerance in older B/W mice under appropriate conditions. These results suggest that the adult B/W thymus may be functionally deficient relative to the young thymus. Neonatal thymectomy accelerated the autoimmune disease, suggesting that the young B/W thymus has some normal regulatory function. Thymectomy at 6 weeks of age did not accelerate autoimmunity. A young B/W thymus, but not an older B/W thymus reversed the accelerated disease of neonatally thymectomized B/W mice. This again suggests that the older B/W thymus is functionally deficient.