We have developed an improved method for measuring the filamentous (F) actin content of human blood polymorphonuclear leukocytes (PMNs). The essential feature of the method is the immediate fixation of the F-actin cytoskeleton. Fresh whole blood (100 µl) is shock-cooled by the addition of 1.0 ml of a mixture of 18.75% glycerol and 5% formaldehyde in phosphate buffer pre-cooled to –8°C and subsequently fixed at 4°C for 15 min. After lysis in distilled water and removal of the red blood cells by centrifugation, the F-actin cytoskeleton of the PMNs is stained with fluorescein isothiocyanate (FITC)-phalloidin and quantified by means of flow cytometry. In healthy test subjects, PMN stimulation by the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (FMLP) for 20 s resulted in a significantly increased F-actin assembly, while in patients with multiple organ failure, two subpopulations arose: one with greater F-actin content and a second with lower F-actin content in comparison with the unstimulated blood sample. This simple and fast method may be a useful tool in basic and clinical research.