SUMMARYResults of this study indicate that the mixed lymphocyte reaction (MLR) can be used to monitor the influence of adult thymectomy as early as 4 weeks following thymus removal. Beginning 16 weeks postthymectomy, the MLR was capable of detecting the recovery of competence in animals grafted with syngeneic thymus tissue. It is suggested, therefore, that the MLR may serve as a sensitive, quantitative, and relatively accurate test for determining the immunological status of lymphoid compartments following adult thymectomy