Performance criteria for manual differential leukocyte counting are tentatively set following analysis of the CAP [College of American Pathologists] interlaboratory survey programs in hematology. Intermediate levels of discrimination of 5 normal leukocyte types (neutrophils, eosinophils, basophils, lymphocytes and monocytes) can be achieved by almost all laboratories. The most sophisticated level (differentiating band from segmented neutrophils and reactive from normal lymphocytes) is still somewhat controversial. Sampling and other variables when doing 100-cell differential counts at the intermediate level of discrimination are quite predictable, and CAP Survey results closely parallel the predicted results when uniform wedge or centrifuged smears stained in an acceptable manner are used. Automated differential counters should be expected to perform at a level of discrimination at least as good as that of human examiners.