The growth of human smooth muscle cells in culture is inhibited by acetylsalicylic acid (ASA). In comparison to control, the proliferation of cells treated with 270 mg/L lysinmono(acetylsalicylate)/30 mg/L glycine was inhibited by 50-90% under different culture conditions. Cell numbers per well (control vs. treated) were as follows: (a) 470,500 +/- 55,890 vs. 24,750 +/- 4,030 (p < 0.002) after 6 days in the presence of 10% fetal calf serum (FCS), (b) 160,500 +/- 9,920 vs. 74,000 (p < 0.001) after 8 days in the presence of 10% human serum; and (c) 387,000 +/- 29,420 vs. 35,250 +/- 1,110 (p < 0.001) after 8 days in the presence of 5% FCS. Significant inhibition of growth by lysinmono(acetylsalicylate) at 90 mg/L was noted only for cultures grown with 10% FCS. Lower concentrations of this drug were ineffective under all culture conditions. Higher dosages of ASA, which would prevent not only platelet aggregation but also smooth muscle cell growth, may therefore be indicated in therapy of patients who undergo percutaneous transluminal coronary angioplasty (PTCA) or coronary artery transplantation.