The assessment of qualitative leukocyte changes is rarely useful to the clinician in making a precise diagnosis of illness unless a quantitative leukocyte abnormality also exists. This may, however, soon change. Current technology, albeit in the setting of malignancy, permits lymphoid subset identification, which has proved to be of immense clinical importance. One may expect that new reagents detecting other leukocyte subsets will yield equally exciting information about nonmalignant diseases. If this technology is to be of widespread clinical value, then standardization of reagents, equipment and technologist performance must parallel progress in antigen detection and disease definition. A model for the setting of such standard can be found in the systematic approach used by the NCCLS for the evaluation of leukocyte counting devices.