Guinea pigs rendered hypersensitive to tuberculin 2 to 3 weeks (early stage) or 10 to 14 weeks (late stage) after sensitization with complete Freund's adjuvant could be completely desensitized by a single or double injections of a sufficient amount of PPD (purified protein derivative). Lymph node cells from such desensitized animals 48 to 72 hr after the challenge showed a considerable reduction of the ability to produce blastogenic factor and skin reactive factor upon PPD-stimulation, whereas the macrophage migration inhibitory factor activity remained still unaffected. As regards antigen-induced 3H-thymidine incorporation in vitro, lymph node cells from animals desensitized in the late stage after sensitization showed no substantial reduction in the degree of enhanced DNA synthesis upon PPD-stimulation, although the desensitization in the early stage resulted in a significant loss of this activity. These results suggest the possibility that desensitizing challenge does not bring about a uniform and regular effect on every lymphocyte subpopulations relevant to different functions and, in addition, indicate that there are some exceptions to the compartmentalization concept of antigen-reactive lymphocytes.