Complete desensitization to tuberculin skin-testing (anergy) was produced in cattle by repeated intravenous injections of living BCG organisms into animals sensitized by a prior subcutaneous dose of BCG. Two levels of desensitization were produced; complete desensitization following 10 i. v. doses and partial desensitization following 5 i. v. doses of 100 mg BCG. Intradermal tuberculin testing at the end of the experiment stimulated the appearance of reactive blood lymphocytes in sensitized cattle as measured by in vitro ³H-thymidine uptake. The cattle which were partially desensitized showed this response but the completely desensitized cattle did not. Reactivity of blood lymphocytes in vitro to PHA and Brucella abortus antigen was not depressed in the anergic cattle. Serum antibody titres to BCG polysaccharide, PPD, or whole BCG organisms showed remarkably little change during the i. v. desensitizing injections of BCG. Differential blood leucocyte counts also remained within normal limits during this period. The production of MIF by blood lymphocytes from the anergic cattle appeared to be unimpaired. Using ³H-thymidine uptake by lymphocytes from sensitized cattle, it was found that serum from desensitized cattle did not inhibit lymphocyte stimulation with tuberculin. It was concluded that the lack of tuberculin-sensitized lymphocytes in the blood of anergic cattle may have been due to their removal from the recirculating pool and their continued suppression in lymphoid tissue.