Cultured human atherosclerotic plaque smooth muscle cells retain transforming potential and display enhanced expression of the myc protooncogene. The proliferation of vascular smooth muscle cells (SMC) is critical to atherosclerotic plaque formation. The monoclonal hypothesis proposes that the stimulus for this SMC proliferation is a mutational event. Here we describe a procedure for growing human plaque smooth muscle cells (p-SMC) in culture. We show that p-SMCs derived from two patients differ from SMC cultured from normal vascular tissue in expression of the protooncogene myc. One p-SMC strain was extensively characterized; these diploid, karyotypically normal cells have a finite life span in culture. Ultrastructural examination revealed two populations, one with classic contractile SMC appearance, the other, modulated to a synthetic state. Northern blotting showed a 2- to 6-fold and a 6- to 11-fold enhanced expression of myc by p-SMC, compared to SMC derived from healthy human aorta (HA-SMC) and saphenous vein (HV-SMC), respectively. In contrast, the p-SMC and HV-SMC expressed similar levels of message for the genes N-myc, L-myc, Ha-ras, fos, sis, myb, LDL receptor, EGF receptor, IGF I receptor, IGF II, and HMG CoA reductase. Finally, although p-SMCs are not tumorigenic, DNA isolated from these cells is positive in the transfection-nude mouse tumor assay. Myc, however, does not appear to be the transforming gene because no newly introduced human myc gene was detected in the p-SMC-associated nude mouse tumor. Thus human atherosclerotic p-SMCs possess both an activated myc gene and a transforming gene that is retained throughout many cell passages. 