The association of IL-1β and fibrinogen characterized using soluble ¹²⁵I-radiolabeled IL-1β and fibrinogen immobilized on Sepharose beads. (A) Binding of IL-1β to fibrinogen. ¹²⁵I-IL-1β was incubated with fibrinogen immobilized on Sepharose beads, and the amount of bound protein was determined as radioactivity associated with the beads following centrifugation and washing. Nonspecific binding was determined in the same way in the presence of a 10-fold molar excess of unlabeled IL-1β. Specific binding was calculated by subtracting the nonspecific from the total bound. (B) Scatchard plot. The best fit of the data was determined by analysis using the Ligand program²⁹  and suggests a single binding site.