Thymic adherent cells were isolated after their enrichment on density gradients. The predominant cell type found was the macrophage, as determined by morphology, surface receptors for Fc and C3, phagocytosis and esterase activity. There was, in addition, a minor fraction of cells with a distinctive dendritic morphology. These dendritic cells had surface properties similar to macrophages but limited phagocytic capacity. Approximately 50% of thymic adherent cells bear Ia antigens detected by immunofluorescence by using either A.TH anti-A.TL or monoclonal anti-I-A antibodies. These cells were also found to be an extremely effective antigen-presenting source for macrophage-depleted immune T cells, supporting the idea that the Ia antigens detected are of functional significance. Our data indicate that the macrophage is the predominant adherent cell type in general, and the principal Ia-bearing cell in particular, isolated by either physical disruption of the thymus or by collagenase dissociation of thymis stroma.