Summary Spontaneous flare-ups at the site of application of a single patch-test of krameria, left in contact with the skin for a period of from one to seven days, occurred in 18 subjects on from the 8th to the 21st day following the initial day of application of the excitant. In 19 subjects, in whom from 2 to 5 applications of krameria, made at intervals of one or more weeks, were needed to produce sensitivity, a flare-up at one previously negative site occurred, and in 4 of these subjects, there were flare-ups at two sites. These sites, moreover, remained inactive for from 10 to 43 days prior to their flare-up. This period was referred to as the inactive phase. The flare-up at sites always occurred in the reverse order of applications to those sites with the excitant and was dependent on the concentration of excitant remaining at these areas. The flare-up could thus occur as a result of a single patchtest contact with an active excitant of contact-dermatitis such as krameria. When more than one application of excitant was needed to induce sensitivity, the appearance of the flare-up seemed to indicate that each successive application of the excitant produced some immunological alteration, and that the sum-total of these stimuli evoked the final state of hypersensitiveness. The occurrence of the flare-up in a subject is, therefore, to be regarded as an indication of the active sensitization of the subject to the excitant.