Thirty patients of heart valve replacement were randomly divided into two groups, one as Radix Salviae Miltiorrhizae (RSM) treatment group and the other as control group. Blood samples were taken from right atrium (RA) and left atrium (LA) before cardiopulmonary bypass operation (CPB) and 10 min, 45 min, 2 hours, 6 hours after the removal of the aortic cross-clamp to observe the level of peroxidation product and the changes in leucocyte count. The results showed that the number of leucocyte in LA was smaller than that in RA and the amount of peroxidation product in LA was higher than that in RA (P < 0.01-0.05) during the early phase of the lung reperfusion in control group. There was no significant difference (P > 0.05) in the amount of peroxidation product and the number of leucocyte between LA and RA in RSM treatment group. The results demonstrated that RSM could markedly prevent the lung leucocyte aggregation and reduce the production of lung free radical products. It suggested that the effect of RSM of reducing the production of lung free radical products was related to its effect of preventing the lung leucocyte aggregation.