Connective tissue-activating peptide-III (CTAP-III) is a 9 kd platelet alpha-granule-derived growth factor. It stimulates the synthesis of DNA, hyaluronic acid, glycosaminoglycans, and proteoglycan core protein in human fibroblasts. Human mononuclear cell-derived proteases have been previously demonstrated to digest the N-terminal 15 residues of CTAP-III (total, 85 residues) to produce neutrophil-activating peptide-2 (NAP-2). CTAP-III and NAP-2 belong to a class of proteins (platelet factor 4, interleukin-8/NAP-1, etc.) associated with inflammation and wound repair. In our efforts to purify human mononuclear cells and platelet-derived histamine-releasing factors, we had previously discovered that mixtures of CTAP-III and NAP-2 released histamine from human basophils. We have now developed simple protocols for the purification of CTAP-III and NAP-2, independently, from calcium ionophore (A23187)-stimulated platelet supernatants by affinity chromatography and have established their identity by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and N-terminal sequence analysis. Each of these related histamine between 2 and 10 micrograms/ml, a range identical to that obtained with CTAP-III/NAP-2 mixtures that we reported earlier. Thus, our data suggest that CTAP-III and NAP-2 independently release histamine from human basophils in dose ranges similar to ranges required for fibroblast stimulation by each.