Effect of paraformaldehyde fixation of cells on PF-4–mediated PMN adhesion and exocytosis. Either neutrophils (○) or EC (▾) were fixed with 3% paraformaldehyde before stimulation with increasing concentrations of PF-4 or both cell types were left unfixed (•). Adhesion and exocytosis were determined as described in the legend to Fig 1, except that with fixed PMN the number of adherent cells was determined by cell counting. Assay backgrounds (adhesion, from 6.1% to 9.9%; exoctosis, 0.2% to 2.0%, depending on the respective cell type fixed) were subtracted. Data are given as the mean ± SD of 3 independent experiments.