1. Seven patients submitted to myocardial revascularization surgery with cardiopulmonary bypass were studied. Blood samples were obtained immediately before and 24 h after surgery. The parameters studied were the production of platelet activating factor (PAF-acether) and superoxide anion, cellular beta-glucuronidase activity as well as polymorphonuclear cell (PMN) and platelet counts. 2. Twenty-four h after surgery, there was a 54% decrease in platelet number (P less than 0.005), a 121% increase in PMN number (P less than 0.005), a 353% increase in PAF-acether (P less than 0.01), a 211% increase in superoxide anion (O2-) and a 104% increase in beta-glucuronidase (P less than 0.05) levels when compared with the pre-surgery levels. 3. The present results indicate that PMN are more reactive after surgery with cardiopulmonary bypass.