Concentration-dependent increase in the level of tyrosine phosphorylation of the p85 subunit of PI3-kinase in GM-CSF–treated neutrophils. The cells were stimulated with 0.04, 0.4, and 4 nmol/L of GM-CSF for 15 minutes at 37°C, lysed under denaturing conditions, and the immunoprecipitations were performed using agarose-conjugated antiphosphotyrosine antibodies. The p85 subunit of PI3-kinase was revealed by blotting with specific anti-p85 antibodies. The densities of the bands were quantified by densitometry. Statistical significance of the difference from control, unstimulated, cells was tested using unpaired Student's t-test. Mean ± SEM of three independent experiments.