Intercellular adhesion molecule-1 (ICAM-1, CD54) is an important early marker of immune activation and response. Evidence on its role has come from immunohistological staining of tissues, since no free circulating ICAM-1 has been detected. By means of monoclonal antibodies against ICAM-1 and a sensitive chemiluminescence technique, free circulating ICAM-1 was detected in serum from sixteen healthy young volunteers. The concentrations varied among the subjects. Non-denaturing gel separation methods showed that ICAM-1 circulates in at least three isoforms, the proportions of which also varied. These findings have important implications for the investigation, diagnosis, and therapeutic monitoring of various inflammatory, neoplastic, and immune disorders.