Plasma cells are readily identified microscopically after immunostaining for the CD138 antigen, and CD138-stained bone marrow core biopsy sections have proved superior to Giemsa-stained aspirate smears and hematoxylineosin (H&E) sections for determining plasma cell percentages in marrow. The CD138-stained plasma cell percentage is generally obtained by visual estimation, after viewing the entire section. We propose an alternative method, in which the microscopist initially views the immunostained section under low- and medium-power magnification, then selects a single high-power field in which the relative concentration of plasma cells appears most representative of the section as a whole, and performs a manual differential count of plasma cells in that field. On 44 CD138-stained core biopsy specimens from patients with plasma cell myeloma, selected area counting provided plasma cell percentage values that were more closely correlated with total section plasma cell area, determined by computerized image cytometry, than were visual estimates. This simple method requires only slightly more time than visual appraisal, does not require extensive training or experience, and appears to provide a more accurate estimate of the plasma cell population within the entire specimen.