A new neutrophil-activating peptide, termed ENA-78, was identified in the conditioned media of stimulated human type II epithelial cell line A549. In response to stimulation with either interleukin 1 beta (IL-1 beta) or tumor necrosis factor alpha (TNF-alpha), ENA-78 was produced and secreted concomitantly with IL-8, GRO alpha, and GRO gamma. ENA-78 consists of 78 amino acids [sequence; see text] and has a molecular weight of 8,357. It has four cysteines positioned identically to those of IL-8 and analogues, and thus belongs to the CXC family of peptides. ENA-78 is related to neutrophil-activating peptide 2 (NAP-2) and GRO alpha (sequence identity, 53% and 52%, respectively) and IL-8 (22% identity). Like NAP-2 and GRO alpha, ENA-78 stimulates neutrophils, inducing chemotaxis, a rise in intracellular free calcium and exocytosis. Cross-desensitization experiments indicate that ENA-78 acts through the same type of receptors as IL-8, NAP-2, and GRO alpha.