A new generation hematology analyzer, Abbott CELL-DYN 4000 (CD 4000), capable of providing 26 parameters, including fully automated reticulocyte, nucleated RBC, blast, band, and immature granulocyte, and variant lymphocyte counts, was evaluated by using the National Committee for Clinical Laboratory Standards H20-A protocol and compared with the Bayer-Technicon H-2 analyzer, which is used routinely in our laboratory. A lipid interference experiment and a sample aging study also were performed. Linearity, carryover, and precision were within the limits established by the manufacturer, and satisfactory agreement was found with the H-2 analyzer. The evaluation of leukocyte differential counts indicated an excellent correlation with the manual reference method for neutrophils and lymphocytes, a good correlation for monocytes and eosinophils, and a poor correlation for basophils in samples with low counts; for basophil counts of 2% or higher, we found an improvement of the correlation coefficient. In the lipid interference experiment, only hemoglobin determination was influenced significantly on the CD 4000, but by using a new Abbott hemoglobin reagent, the interference was eliminated. The CBC and differential counts were stable and reportable up to at least 24 hours. Intrasample viability information on leukocytes provided a quality check on each individual specimen.