Isolated arterial smooth-muscle cells (SMCs) cultured in medium containing whole blood serum or plasma-derived serum undergo modulation from a contractile to a synthetic phenotype. This process includes the loss of myofilaments and cessation of the ability to contract. lnstead, an extensive rough endoplasmic reticulum and a large Golgi complex are formed and, if properly stimulated, the cells start to proliferate actively and to produce extracellular-matrix components. In vivo, a similar change in the differentiated properties of SMCs appears to be an early key event in atherogenesis. The purpose of the present investigation was to try to identify plasma components that promote the modulation of the smooth-muscle phenotype. SMCs were enzymatically isolated from rat aorta and cultured in a defined, serum-free medium. The phenotypic state of the cells was determined by transmission electron microscopy, and their growth status was followed by 3H-thymidine autoradiography and cell counting. Under these conditions, Cohn fractions I (fibrinogen) and V (albumin) were found to partially support cell attachment and transition from the contractile to the synthetic phenotype, whereas fractions II–III and IV (globulins) were inactive in this respect. Analysis on adsorptive columns of gelatin Sepharo-se 4B indicated that Cohn fraction I, but not fraction V, contained fibronectin, an adhesive protein that is present in plasma and binds to fibrinogen. When seeded on a substrate of plasma fibronectin, the cells attached with high efficiency and modulated into the synthetic phenotype at a rate similar to that observed in serumcontaining medium. In the absence of exogenous mitogens, the structural transformation of the cells was not accompanied by a proliferative response. However, when exposed to platelet-derived growth factor or serum, the cells promptly started to synthesize DNA and divide. These findings indicate that plasma fibronectin is a major determinant of the phenotypic properties of arterial SMCs, and they confirm the view that phenotypic modulation is necessary, but is itself not sufficient, for the initiation of SMC growth. The experimental system outlined here should make it possible to study the control of SMC phenotype and growth separately and under defined conditions.