We studied the adaptation of a filtration instrument to an assay for cell migration experiments. The commercially available Nuclepore filter holders were modified so that isolated human polymorphonuclear leucocytes (PMNs) were allowed to penetrate a porous filter membrane and to enter a compartment under the membrane. The total number of PMNs which had passed the pores during an hour of incubation could be counted. In this way we carried out experiments using Shigella sonnei lipopolysaccharide and dilutions of E. coli culture supernatant as chemoattractants for healthy human PMNs. These studies showed that the method can distinguish between random and directed movements of PMNs and it is sensitive to the concentrations of chemoattractant. Furthermore, the data obtained using PMNs from the same subject on different days seem to be comparable.