The present paper examines evidence for the identity of low molecular weight eosinophil chemotactic factor (ECF) and of leukotriene B4 (LTB4) and its metabolites. Total congruity between the two entities is found regarding (1) cells of origin; (2) conditions for in vitro generation and pharmacological modulation; (3) physiochemical properties; (4) in vitro chemotaxis towards human monocytes, fibroblasts and guinea pig eosinophils; (5) in vivo activities in humans, and (6) occurrence of the factors in various dermatological diseases. Quantitative differences were observed only for in vitro neutrophil migration which may be due to neutrophil chemotactic mono-HETEs and possibly platelet activating factor in the ECF preparations. The name ECF should therefore be replaced by LTB4 and its metabolites, as has happened for SRS which is now called LTC4/D4.