A radioassay of granulocyte chemotaxis employing chromium-51 labeled cells and a double micropore filter system is presented. In comparison to the conventional Boyden chamber technique, this method offers significant advantages including: a) elimination of tedious filter staining and microscope counting resulting in a considerable saving in laboratory time, b) elimination of observer subjectivity, c) excellent sensitivity over a wide range of cell responses with no increase in variability, d) ability to monitor cellular events in all compartments of the chemotaxis chamber and e) a means for standardization of neutrophil chemotaxis among different laboratories.