Binding of ¹²⁵I-IL-1β to fibrin monomer was found to be of high affinity. (A) Binding of IL-1β to fibrin monomer. Fibrinogen was immobilized on Sepharose beads and then converted to fibrin monomer by incubation with thrombin. ¹²⁵I-IL-1β was incubated with the beads, and bound and free ligands were then separated by centrifugation. Nonspecific binding was measured in the presence of a 10-fold molar excess of unlabeled IL-1β, and specific binding was determined by subtraction of nonspecific from total binding. Each point represents the mean of 3 different experiments. (B) Scatchard plot. The best fit of the data was determined using the Ligand program²⁹  and indicated the presence of single binding site.