Cell-mediated immune reactivity was studied in 23 patients with mycosis fungoides and Sézary’s syndrome by a semi-quantitative 2,4-dinitrochlorobenzene (DNCB) sensibilization test and the reaction to intracutaneous microbial antigens. Anergy to DNCB was observed in 8 patients, 5 had a markedly depressed DNCB score, 6 showed negative reactions to the microbial antigens. A defect in antigen recognition is suggested.