Competition assays comparing the relative inhibition of binding of biotinylated S5A8 by unlabeled S5A8, chS5A8, and chS5A8–IL-2 Abs as well as two monovalent anti-Id Abs, scFvS5A8–IL-2 and S5A8 Fab fragment. Microtiter plates were coated with purified 38C13 Id. Serial dilution of each unlabeled competitor anti-Id Abs were added to each well together with a fixed amount of biotinylated S5A8 and incubated overnight at 4°C. The bound biotinylated S5A8 were then quantitated. The percentage of inhibition values represent the mean of triplicate trials. SE values were within 10% of the mean.