A histamine liberator, compound 48/80, was administered by continuous infusion for two hours in patients with atopic dermatitis, chronic urticaria, and alopecia areata. The time for the appearance of urticaria varied widely. Patients with chronic urticaria tolerated the infusion best. In cases of alopecia areata the urticaria first appeared in the bald areas of the scalp, which contained a large number of mast cells. In all patients the white cells of the blood increased after infusion of 48/80, with appearance of juvenile leukocytes. Several white cells were atypical, with homogenous, crescent-shaped nuclei pushed towards the periphery. The eosinophils showed extrusopm of granules. In nine of the 14 patients the number of basophils increased during infusion, owing to the appearance of young, nondegranulated basophils. Pretreatment with heparin gave the best protection against the effects of 48/80.