A simple method was devised for counting small numbers (10(4)-10(6)) of adherent mononuclear phagocytes, including populations containing multinucleated giant cells, which often arise during cultivation of human blood monocytes. Coverslips with adherent cells were transferred into small volumes (50-200 microliters) of 0.1 M citric acid, pH 2.2, containing 0.05% naphthol blue black and 1.0% of either Triton X-100 or Cetavlon. Triton X-100 was adequate for use with monocytes and macrophages from early cultures. However, Cetavlon was preferable for use with older cultures of adherent human mononuclear cells in order to prevent aggregation of the nuclei from giant cells. When multinucleated cells were present, separately stained coverslips were inspected to determine the mean number of nuclei per cell. This value, together with the number of nuclei per coverslip, permitted calculation of the number of cells per coverslip. The latter value is not readily derived from measurements of protein or DNA content in populations containing multinucleated giant cells. This counting method was simpler and more sensitive than several previously reported methods for enumerating adherent macrophages.