An estimate of the observer classification error was obtained for six classes of peripheral blood leukocytes: lymphocytes, segmented and band neutrophils, eosinophils, basophils, and monocytes. The images, in the form of colored photomicrograph transparencies, were classified independently by 11 senior hematology technicians. The consensus was used to obtain a classification “truth” for each image, and each classification result was then compared with this “true” classification to obtain error rates. When the segmented and band neutrophils were lumped together as one class, the resulting average observer error per class for the consolidated five classes was approximately 1%. The error in classifying segmented versus band neutrophils was estimated at about 7% per class for those observers with the least systematic bias toward naming one of these cell types preferentially.