GM-CSF–induced activation of PI3-kinase is dependent on tyrosine kinase activity. Neutrophils were pretreated with erbstatin (10 μg/mL) or diluent (0.1% DMSO) for 1 hour at 37°C. This was followed by treatment with GM-CSF or diluent (0.01% BSA) for 15 minutes at 37°C. Lysis was conducted under nondenaturing conditions and immunoprecipitation was performed using anti-p85 polyclonal antibodies. An in vitro kinase assay was conducted using phosphatidylinositol as substrate. Lipids were separated by TLC and revealed by autoradiography and quantified by densitometry. Statistical significance was evaluated using unpaired Student's t-test. Mean ± SEM of four independent experiments.