The effect of cigarette smoking on peroxidation of plasma low density lipoprotein (LDL) was studied in 16 smokers aged 23-56 years; 12 nonsmokers of similar age served as controls. The smokers were asked to refrain from smoking 24-40 h prior to testing. Peroxidation was assessed by determination of thiobarbituric acid reactive substances (TBARS) in fresh plasma and LDL and by an increase in 125I-LDL metabolism by peritoneal macrophages. There was no difference in TBARS in freshly prepared plasma or LDL of smokers and nonsmokers. However, LDL of smokers, conditioned by incubation with bovine aortic smooth muscle cells (SMC), had 2-fold or higher TBARS values when compared to SMC conditioned LDL of nonsmokers. 2-4-fold higher TBARS values were seen also in SMC conditioned LDL when the comparison was made between LDL isolated from plasma before smoking of 6-7 cigarettes (time 0), and LDL of the same individual isolated from plasma 90 min thereafter. The metabolism of SMC conditioned 125I-LDL by peritoneal macrophages was examined; LDL isolated from plasma of smokers at time 0 was metabolized twice as avidly as LDL of nonsmokers and a further increase was seen with LDL isolated 90 min after acute smoking. The present results indicate that cigarette smoking renders plasma LDL more susceptible to subsequent peroxidative modification by cellular elements.