Besides its effect on bone marrow progenitors, GM-CSF is able to interact with mature cells such as granulocytes which express GM-CSF receptors at their surface. Using an incubation period of 90 min, which corresponds to the maximum expression of GM-CSF receptors, we studied oxidative metabolism, random migration and chemotaxis as well as elastase release. Elastase was chosen as a marker for primary granules. We observed inhibition of chemotaxis and random migration, and stimulation of oxidative metabolism as well as elastase release in presence of rh GM-CSF. These different effects were dependent on the dosage of rh GM-CSF applied.