In delayed-type hypersensitivity reactions, cytokine-mediated cell migration leads to localized accumulation of neutrophils and mononuclear cells over 4-48 h. In contrast to transient (2-6 h) responses elicited by other chemotactic factors, earlier studies indicated that a chemotactic activity previously described in our laboratory elicited skin test responses over 24 h, identical to those induced by injection of antigen into a sensitized test subject. We have isolated this factor, a 10.3-kDa chemotactic protein designated CP-10, from supernatants of activated murine spleen cells. Purification to homogeneity was achieved using affinity chromatography on iminodiacetic acid-immobilized copper and cation-exchange, mixed mode (cation exchange/metal affinity), reversed-phase, and size-exclusion high performance liquid chromatography. CP-10 had maximal chemotactic activity for neutrophils at 10(-13) M. The 76-amino acid sequence, obtained by automated N-terminal microsequence analysis of native CP-10, and fragments derived from trypsin digestion and cyanogen bromide cleavage indicated no sequence identity with any known cytokine or chemotactic factor but revealed up to 55% sequence homology with S100, Ca2(+)-binding proteins. CP-10 appears to be the first protein of this family with a well defined function affecting cell migration, and its biological potency suggests an important role for this cytokine in cellular immune reactions.