Blood was obtained before and after ten healthy male nonsmokers sat for 20 minutes in open hospital corridors beside men who were already there smoking by their own initiative. Mean values before and after passive smoking were 0.87 and 0.78 for the platelet aggregate ratio, 2.8 and 3.7 per counting chamber for the endothelial cell count, 0 and 2.8 ng/mL for the plasma nicotine concentration, and 0.9% and 1.3% for the carboxyhemoglobin level. No variable changed significantly during control periods in which the subjects sat in a room where smoking was prohibited. Passive exposure to tobacco smoke affected the endothelial cell count and platelet aggregate ratio in a manner similar to that previously observed with active smoking.