During experiments aiming at the generation of monoclonal antibodies against native human interleukin 2 an antibody of different specificity was obtained, recognizing a polypeptide contaminant within the antigen preparation used for immunization. This antigen was shown to represent a release protein from human blood platelets. Amino acid sequence analysis of the immunopurified antigen revealed its identity as beta-thromboglobulin antigen. Depending on the source of antigen (freshly lysed platelets, platelet containing cell culture supernatants) various forms of the polypeptide, differing in the degree of N-terminal truncation, were found. Beta-thromboglobulin antigen preparations differing in peptide composition also had different capacities for modulating spontaneous as well as Fc-receptor dependent chemiluminescence of human monocytes and granulocytes. In contrast to former reports, no mitogenic activity for human dermal fibroblasts was found with beta-TG antigen (CTAP III) alone, but only in combination with human interleukin 1 and heparin, the three molecules acting synergistically. These findings indicate that beta-TG antigen could play a functional role in linking the blood clotting system to the immune system.