The glomerular mesangium is composed of matrix material and at least two cell types. One is a bone marrow-derived phagocyte and the other is a smooth muscle-like cell. The phagocytic cell represents approximately 3-7% of the total mesangial cell population. The other, more abundant, cell type appears to be contractile and therefore has been proposed to play a role in regulating the surface area for filtration, one component of the ultrafiltration coefficient, Kf. In this review we discuss the contractile properties of cultured mesangial cells as well as the phenotypic alterations that lead to loss of isotonic contraction after prolonged culture.