The antigenic composition of the rat kidney was studied by indirect immunofluorescence and immunoelectron microscopy using monoclonal antibodies against Thy-1.1 (OX7 and ER Ly 1), Thy-1.2 (30-H12), and W3/13 as well as rabbit antisera against rat brain or thymic blasts. The anti-Thy-1.1 reagents caused pronounced staining of the glomerular mesangium as well as a weak stain around some of the medullary collecting ducts and/or vessels. The rabbit antisera contained additional antibodies that bound to renal basement membranes and smooth muscle cells. This was confirmed by blocking and absorption experiments. The presence of Thy-1.1 in glomeruli was further demonstrated by in vivo perfusion studies with OX7 and rabbit anti-thymic blast serum. Immunoelectron microscopy of isolated glomeruli showed binding of OX7 antibodies to mesangial cells and the mesangial matrix. It is concluded that in rat kidneys the Thy-1.1 antigen is expressed on glomerular mesangial cells.