BION-1 blocks high-affinity binding of IL-5, GM-CSF, and IL-3 to human eosinophils. Human eosinophils (1.8 × 10⁶) were preincubated either alone (□) or in the presence of 1 μmol/L BION-1 MoAb (▪) or an isotype IgG₁-matched control antibody (▧) before the addition of radiolabeled cytokine. Nonspecific binding was in each case less than 1% of total counts added. Each point is the mean of duplicate determinations and error bars represent 1 standard deviation.