Leucocyte membrane fluidity affects cell deformability as well as the accessibility of receptors and the degree of their exposure on the membrane. These effects modulate the subsequent cellular responsiveness. We have assayed membrane fluidity of polymorphonuclear leucocytes of children, using an excimer-forming lipid technique in flow cytometry, and evaluated its relation to their chemotaxis to a formyl peptide. We report that membrane fluidity and chemotaxis of polymorphonuclear leucocytes are increased with age. These findings may have important implications for the physiological processes in polymorphonuclear leucocyte motility during childhood.