We have compared phenotypic and functional properties of surgically derived adult human microglia to autologous and allogenic peripheral blood-derived monocytes and to astrocytes derived from the same surgical resection. We found that microglia differed from peripheral blood monocytes with respect to adhesion properties and survival rates in vitro. Microglia, similar to resident macrophages in different tissues, expressed many but not all (CD4, Leu-M3, non-specific esterase) monocyte/macrophage associated markers tested, a pattern similar to that of terminally differentiated cells of this lineage. As with other human tissue macrophages, but in contrast to astrocytes, microglia did not undergo DNA synthesis in vitro, assessed using BrdU incorporation. Under basal culture conditions the majority of microglia of all morphologic subtypes (ameboid, bipolar, ramified) expressed MHC class II molecules; by flow cytometric analysis, mean fluorescence intensity of these cells was less than that of blood monocytes (relative to isotype control). In vitro MHC class II antigen expression on microglia, under basal and interferon gamma activating conditions, was greater than on astrocytes. Freshly derived T cells cultured with 1-10% autologous microglia plus Candida albicans underwent active proliferation, indicating the functional capacity of the microglia to serve as antigen-presenting cells.