Automated leukocyte differential counting has resulted in a marked improvement of the precision of leukocyte subclass enumeration. With advancement of this technology, algorithms have been developed to identify samples that require manual microscopic review. Complex flagging algorithms permit acceptable false-positive rates with effective use of microscopic review to identify significant morphologic abnormalities. Current literature indicates that the leukocyte differential is a poor case-finding index in both outpatient and inpatient populations. Development of highly automated technology has led to test over-utilization because of ease of performance despite recognized limitations in identifying clinically significant abnormalities. Enumeration of hematopoietic stem cells in harvested peripheral blood or bone marrow is critical for determining whether transplanted cells will produce adequate engraftment. Hematopoietic precursors express the CD34 antigen, are present as rare events in unmobilized blood, and are found in increased numbers after mobilization with chemotherapy with or without G-CSF. Flow cytometric detection of CD34+ cells is more rapid and precise than cell culture techniques and results are available in time to determine if further cell harvesting procedures will be necessary. CD34+ cell concentrations correlate with granulocyte-macrophage colony-forming unit counts as well as with time to engraftment. Although intralaboratory precision in measuring these infrequent events may be good, comparisons between different laboratories using aliquots of the same sample have yielded more variable results. Problems in determining precise and reproducible CD34+ cell concentrations may be due to nonspecific antibody binding, insufficient signal intensity of true positive events, differences in monoclonal antibodies and their fluorochrome conjugates, or failure of the total absolute leukocyte count to accurately reflect the population used for CD34 analysis.