An IFN-gamma-inducible protein, IP-10, has previously been described to belong to a gene family of chemotactic and mitogenic proteins, associated with inflammation and proliferation. Biochemical characterization of this predicted protein has been pursued through the development of polyclonal monospecific antisera to recombinant protein and synthetic peptides. These reagents establish that the IP-10 protein is secreted from a variety of cells (endothelial, monocyte, fibroblast, and keratinocyte) in response to IFN-gamma. Posttranslational processing occurs in the biosynthesis of this protein, resulting in a 6-7-kD species, which may reflect COOH-terminal cleavage. Pulse-chase studies indicate that this processing is a rapid event in the primary cell lines studied, completed in the 30-min labeling period. A model is presented for the processing and secondary structure of this protein. In an accompanying study, Kaplan, et al. using these antisera, demonstrate that the IP-10 protein is associated, in vivo, with a delayed-type hypersensitivity response.