A specific radioimmunoassay has been developed for determination of human granulocyte elastase in blood. THE granulocyte elastase employed as radioiodinated tracer in the assay was inactivated with diisopropylfluorophosphate in order to prevent binding of the tracer to the serum inhibitors alpha2-microglobulin and alpha1-anti-trypsin, while still retaining its immunoreactivity. The labelled tracer showed, however, a pronounced tendency to nonspecific binding to serum proteins such as albumin and alpha2-macroglobulin and also to the Sephadex particles. The binding of the labelled tracer to alpha2-macroglobulin caused a false increase in the immunoreactive granulocyte elastase in serum. But the binding of the labelled tracer and its consequences could be circumvented by increasing the NaCl concentration of the reaction mixtures and/or gel filtration buffers. Freshly drawn normal human serum contains about 135 microgram granulocyte elastase/l measured as diisopropylfluorophosphate-inactivated granulocyte elastase. The results of experiments in which serum was fractionated by Sephadex G-100 gel filtration suggest that essentially all of the immunoreactive material in normal human serum is granulocyte elastase bound by alpha1-antitrypsin. This finding implies that granulocyte elastase is released from the cells in an active form and then rapidly bound by the inhibitors.