Five patients with mycosis fungoides who had developed contact dermatitis to a nitrogen mustard, mechlorethamine hydrochloride, even in low concentrations (1 to 5 mg/100 ml), received daily total-body applications of extremely dilute solutions (0.01 to 0.1 mg/100 ml) of mechlorethamine. The concentrations of the drug were approximately doubled weekly if the patient could tolerate it, or they were raised more slowly if the patient could not. Attempts to desensitize one patient were discontinued since he was unable to tolerate a greater concentration than 1.0 mg/100 ml after trying for one year. Another patient was able to tolerate a concentration of 3 mg/100 ml after three months, at which time his skin had completely cleared and treatment was stopped. Three other patients were desensitized during a period of 8 to 13 months to the point of tolerating the full therapeutic concentration used in our clinic (20 mg/100 ml) without experiencing dermatitis or pruritus.