A factor able to induce an early local inflammation in rabbit skin was detected in the supernatant of mitogen-stimulated human blood leukocytes. The factor was different from IL-1 which, although present in the supernatants, was chemically separable from the factor and induced a late rather than an early skin response. Other biological effects of the principal factor were its in vitro chemotactic effects on granulocytes and its ability to induce rapid granulocytosis upon intravenous injection in rabbits. When tested under the same conditions, IL-1 beta did not act chemotactically and induced granulocytosis at a later time. The factor was purified to homogeneity and identified by electrophoretic mobility as a protein of Mr 6,500. Amino acid sequence analysis revealed the presence of an uncontaminated NH2-terminal sequence identical to a segment of the sequence previously predicted from the cDNA clone (3-10C) copied from an mRNA isolated from human leukocytes and coding for a protein of unknown function. The NH2-terminal sequence of the factor also showed extensive homology to that of the platelet factors beta-thromboglobulin (beta TG) and platelet factor 4 (PF-4). Studies done to identify the cell source of the factor revealed that it was produced by adherent mononuclear cells but not by platelets, while the opposite was true for beta TG.