Human monocytes/macrophages display a variety of cell-surface determinants to which monoclonal antibodies (MAbs) have been produced. These MAbs can be used to characterize phenotypic profiles of monocytes/macrophages to gain insight into their developmental and activation status. The basic and alternate protocols in this unit describe flow cytometry methods for measuring the expression of membrane determinants using directly conjugated or unconjugated MAbs, respectively. Because cells of this lineage display characteristic membrane marker patterns, combinations of MAbs are used in three-color analysis to facilitate immunophenotypic characterization. Detailed information and commercial sources of fluorochrome-labeled MAbs are provided, and suggested combinations of these MAbs for three-color flow cytometry analysis of cell suspensions are discussed.