Synovial lining cells obtained from patients with rheumatoid arthritis or noninflammatory joint diseases were divisible into three populations according to the expression of surface antigens detected by various monoclonal antibodies. A population of cells designated type I was defined by the presence of Ia antigens, Fc receptors, five different monocyte lineage differentiation antigens, and the property of phagocytosis. The greatly increased amounts of both Ia antigens and certain monocyte lineage antigens distinguished these cells from blood monocytes. A second distinctive cell population was non‐phagocytic, occasionally binucleate, and had abundant Ia antigens but lacked IgG Fc receptors, monocyte lineage antigens. B or T lymphocyte antigens, and fibroblast‐associated antigens detected by reagents raised against synovial cells. This population, designated type II, accounted for approximately one‐third of the synovial cells in patients with rheumatoid arthritis but few or no cells in the synovial lining of patients with non‐inflammatory diseases. The Ia‐positive synovial cells with a dendritic morphology were contained in this population. An additional population, designated type III, contained nearly all of the remaining cells and was defined by the presence of antigens expressed primarily on fibroblasts and by the absence of phagocytosis, demonstrable Ia antigens, and four antigens of the monocyte lineage. This population exhibited proliferative capacity, becoming the predominant cell in long‐term cultures. The proportions of each population varied considerably from patient to patient.