Effect of PMA on the stimulation of the activity of lyn induced by GM-CSF in human neutrophils. The cells were simultaneously treated with BSA (0.0.1%), GM-CSF (4 nmol/L), and/or PMA (90 nmol/L) for 15 minutes at 37°C. The cells were then lysed under nondenaturing conditions and lyn was immunoprecipitated as described in Materials and Methods. An in vitro kinase assay using a specific synthetic substrate was conducted as detailed in Materials and Methods. The level of the phosphorylation of the peptide was monitored by liquid scintillation counting. Statistical significance was evaluated using unpaired Student's t-test. Mean ± SEM of four independent experiments.