Chlorocresol was shown to have great potential for contact sensitization in the guinea-pig maximization test at the Day-21 challenge; 2 weeks later, the reactivity was significantly decreased. Cyclophosphamide (250 mg/kg) was interperitoneally injected and used as a modulator for the immune response. When cyclophosphamide was administered 3 days before the first challenge, the decrease in reactivity from Day 21 to Day 35 was reversed. This indicates that the reduction in sensitivity was caused by an immunologic mechanism--probably the formation of suppressor cells. This phenomenon is called the "down regulation" of sensitivity and may operate in eczema patients who have lost patch-test reactivity.