Effect of PMA on the stimulation of the in situ tyrosine phosphorylation 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. (Top) In situ tyrosine phosphorylation of lyn was determined by immunoprecipitating lyn under denaturing conditions from control or stimulated cells and blotting using antiphosphotyrosine antibodies. (Bottom) Mean ± SEM of five independent experiments such as presented in the top panel and quantified by densitometry. Statistical significance was evaluated using unpaired Student's t-test.