In the present study the influence of interleukin 3 and interleukin 5 on the migration of normal eosinophil and neutrophil granulocytes has been investigated. LTB₄, PAF, f‐MLP, C5a and ZAS were used as chemoattractants, and HSA and pooled normal human serum were used as chemokinetic agents. Recombinant human IL‐5 (rh‐IL5) at a concentration of 4 times 10⁻¹² mol/1 was chemotactic for eosinophils, while recombinant mouse IL‐5 (rm‐IL5) attracted both eosinophils and neutrophils. IL‐3 (rh‐IL3) at a concentration around 10⁻¹² mol/1 exerted a priming effect on eosinophil and neutrophil migration, i.e. chemotactic and chemokinetic responses to all agents tested. Human IL‐5 at a concentration of 2 to 20 times 10⁻¹² mol/1 primed the chemotactic and chemokinetic responses of eosinophils to all agents tested. The migration of neutrophils was also primed by rh‐IL5, but at higher concentrations, i.e. around 10⁻¹⁰ mol/1. IL‐5 of mouse origin primed the migration of both eosinophils and neutrophils. In conclusion, IL‐3 primed the migratory function of both eosinophils and neutrophils, while IL‐5 was a more potent primer of eosinophil than of neutrophil migration.