The normal rat glomerulus contains a mesangial population of mononuclear leukocytes, 35 to 70% of which express class II major histocompatibility complex antigen (equivalent to mouse Ia) Schreiner GF, Unanue ER: Lab Invest 51: 515, 1984. Inducing factors for cellular expression of class II antigens in normal leukocytes are unknown. In this population, glomerular leukocyte origin and Ia expression have been studied in kidneys from bone marrow-irradiated Lewis rats transplanted into normal syngeneic recipients. Leukocytes (LC+) and Ia positive (Ia+) cells were enumerated in isolated glomeruli by immunofluorescence using monoclonal antibodies to rat common leukocyte antigen (MRC OX1) and rat Ia antigen (MRC OX3, and OX4). Irradiation to donors caused extreme depletion of glomerular LC+ cells. Twenty-four hours after renal transplantation into normal recipients the glomerular LC+ population was restored to 65% of normal, and by 3 days was complete. In contrast Ia+ cells which were also depleted by irradiation, remained low 24 hours post-transplantation and only reached normal levels after 4 days. Thus, Ia antigen expression is induced in a population of leukocytes derived from circulating mononuclear leukocytes after a residence time of 3 to 4 days in the glomerulus. While the glomerular factors responsible are unknown, these experiments provide the first evidence for in vivo basal Ia induction in a normal resident mononuclear leukocyte population.