A rapid suspension method for alpha-naphthyl acetate esterase staining has been utilized to evaluate monocytes percentages in Ficoll-Paque separated mononuclear cells (MC) from normal individuals. In MC preparations from 24 normal subjects a mean percentage of 8% monocytes has been found, with a range from 2% to 20%. Cell fractions depleted of T-lymphocytes by rosette sedimentation showed 43% of esterase positive cells (range 31-53%), while very few monocytes (2%) appeared to contaminate the preparations of E-rosetting cells. The method for esterase staining of monocytes while they are in suspension is very simple and permits a rapid and accurate evaluation of monocyte percentages in mixed cell preparations.