Preliminary data on the performance of a new semiautomated pattern recognition instrument (Hematrak) in the detection of abnormal nucleated blood cells on peripheral slides are presented. The Hematrak instrument offers performance at least equivalent to the hospital laboratory’s 100-cell differential in detecting nucleated erythrocytes, blasts, immature myelocytes, and combinations of these abnormal cells, even when present in low concentrations. Evaluation of the instrument with regard to the detection of plasma cells and atypical lymphocytes remains uncertain. The problems of both manual and automated differentials in regard to the necessity for improved criteria for cell identification are briefly discussed.