An electronic blood cell counter (Coulter S-plus IV) classifies leukocytes as lymphocytes, large mononuclear cells, or granulocytes based on volume after treatment with a reagent. Correlation coefficients as compared with stained film microscopy are .94 for lymphocytes, .91 for granulocytes, and .49 for mononuclear cells. As a screening tool, it could reduce the number of traditional manual differential cell counts. We devised criteria for identifying those specimens requiring additional visual examination, giving particular attention to "low-density" abnormalities. Over 2,200 specimens were studied. Among 791 specimens obtained primarily from inpatients, the screening procedure yielded 267 true-positive, 298 false-positive, 213 true-negative, and 13 false-negative specimens. The 1,216 specimens obtained primarily from outpatients included 54 true-positive, 234 false-positive, 912 true-negative, and 16 false-negative specimens. All false-negative specimens represented low-density abnormalities. The usefulness of this approach is partially dependent on patient population.