Glomerular mesangial cells were isolated from explants of normal rat whole glomeruli according to the differential growth capacities of the cultured cells. The cultured mesangial cells comprised almost all of the grown cells on day 21 of the primary culture when the subculture could be performed. They contained abundant cytoplasmic microfilaments and actin. The contractile response to vasoactive hormone of both primarily cultured and subcultured mesangial cells was quantified as a decrease in cell surface area. The maximum decrease in surface area in response to 1 nM of angiotensin II was about 30%. This magnitude of contraction is consistent with that observed in cultured rat vascular smooth muscle cells. The mesangial cells did not phagocytose horse-radish peroxidase or sensitized sheep red blood cells, nor did they have binding capacities for Fc or C3b, while the cells derived from explanted glomerulus and probably of blood-born monocyte origin did. We conclude that the most important function of the glomerular mesangial cell is the control of glomerular blood flow and filtration by contraction.