Glomerular cells cultured in fetal calf serum showed a progressive series of morphological changes as the culture aged in vitro. Guinea pig glomerular cells initially contained large intracytoplasmic granules, which are believed to be packets of newly formed extracellular matrix protein molecules. As the cells became older, the granules changed from an extremely electron dense nature to an electron density identical to that of glomerular basement membrane. These granules disappeared from cultured cells with time, and abundant extracellular matrix was produced. Immunochemical studies showed this matrix to be composed at least partially of fibronectin. As the culture aged, the cells became filled with microfilaments which were present just beneath the cell membrane and as large bundles throughout the cytoplasm. At the time that the granules had disappeared and the microfilaments were abundant, the cells became senescent and could no longer be subcultured. A further important feature of the aging glomerular cell in culture was the accumulation of both intracellular and plasma membrane changes. Growth of glomerular cells in defined media containing fibroblast growth factor dramatically modified the behaviour of these cells in vitro and provided important information concerning the role of fetal calf serum in the proliferation of glomerular cells and their production of extracellular matrix.