The occurrence of low molecular weight (less than 500 dalton) eosinophil chemotactic factor (ECF) was studied in the blister fluid of patients with various dermatological diseases by an in vitro chemotaxis method. After chromatographic separation of the test substance on a small Sephadex G 25 column, ECF was demonstrated in blisters of five patients with bullous pemphigoid, in three patients with systemic drug reactions and in one patient with blisters are contact with primula. ECF was not found in blisters of patients with epidermolysis bullosa and herpes gestationis or in DNCB-induced blisters. Fluid obtained from suction blisters was negative in normal controls and in unaffected skin of patients with atopic eczema, but contained ECF in eczematous areas. The demonstration of ECF in tissue fluid suggests that this factor plays a role in the local accumulation of eosinophils at sites of certain inflammatory reactions.