Smooth muscle cells (SMC) were grown from inner media explants of swine aorta and used as a model for studying the role of lipoproteins in atherogenesis. These cultured cells retain the characteristics of SMC through multiple passages. Cell growth curves, in time, were obtained by using standard counting techniques, SMC grew slowly (0.019 cycle/day) in modified Dulbecco-Vogt medium supplemented with 1.5% swine serum. Purified lipoproteins were prepared from three normolipidemic and two hyperlipidemic (cholesterol-fed) swine. When the medium of 84 growth experiments was supplemented with these lipoproteins, SMC growth rate increased linearly with lipoprotein cholesterol concentration up to 10 mg/dl. At 10 mg/dl of lipoprotein cholesterol, very low density lipoproteins (VLDL) increased growth rate 7.2-fold (P less than 0.01); low density lipoproteins (LDL) 5.7-fold (P less than 0.01); high density lipoproteins (HDL2) 3,4-fold (P less than 0.02); and HDLc, and lipoprotein appearing in the hyperlipidemic swine, 3.0-fold (P less than 0.01). Addition of 10% lipoprotein-free serum stimulated growth rate 6.0-fold (P less than 0.01). There was no difference between normo- and hyperlipidemic lipoproteins with respect to cell growth rate. Factors present in the ultracentrifugal bottom, and factors appearing during the platelet release reaction, were shown to contribute to the SMC growth response. Morphological alterations characteristic of intimal foam cells occurred in SMC grown in VLDL at triglyceride levels in excess of 15 mg per 100 ml. Thus there are distinct parallels between SMC response in this model in vitro and atherogenesis in vivo.