CD31 stimulation of eosinophils increases the adhesive function of 4β1 integrin (VLA-4) to fibronectin.⁵¹Cr-labeled eosinophils were preincubated with anti-CD31 MoAb or control mouse IgG (10 μg/mL each) for 10 minutes at 37°C and were then washed. ⁵¹Cr-labeled eosinophils (5 × 10⁵) were then added to the fibronectin-coated well of tissue culture plates and were incubated for 10 minutes at 37°C in the presence or absence of anti–VLA-4 MoAb (10 μg/mL). After unbound cells were washed off, bound cells were lysed by the addition of HBSS containing 1% Triton X-100 and radioactivity of the cells was counted in a gamma counter. Data are the means ± SD for 4 experiments. *Significantly different from the mean value of the control response (*P < .005). **Significantly different from the mean value of the control response of anti-CD31 MoAb-pretreated eosinophils (**P < .005).