The Coulter STKS (Coulter, Hialeah, FL), the Abbott CD3500 (Abbott Diagnostics, Abbott Park, IL), a 400-cell manual differential, and flow cytometry using double-staining with fluorescence-labelled monoclonal antibodies (CD45-FITC and CD14-PE) on a Coulter Epics Profile II were evaluated for precision and accuracy in relative monocyte counting. STKS, CD3500, and Profile II achieved a precision analogous to a 3,542-, 1,835-, and 11,998-cell differential, respectively, demonstrating the superiority of automated methods. Analysis of 156 normal and abnormal samples revealed that the mean relative monocyte counts of the manual differential, CD3500 and Profile II were not significantly different. Only the STKS results showed a positive bias (0.79% +/- 1.65), which was increased in lymphocytic samples. Linear regression between the Profile II as independent viable, and the other techniques yielded acceptable correlation coefficients (STKS: 0.861, CD3500: 0.844, manual differential:0.833). Profile II results were also compared to those of a Becton Dickinson FACScan (Becton Dickinson, Mountain View, CA), which yielded an excellent correlation (r = 0.991) but a slightly smaller relative monocyte count (bias-0.39% +/- 0.60) of the latter. On the basis of these data, the authors recommend the use of monoclonal antibodies as a new reference method, but also indicate the need for further methodological investigations.