Airborne contamination of the wound area and the cardiopulmonary bypass circuit during sham open-heart operations on dogs was studied. The air of the operating room (OR) was contaminated with two typeable bacterial strains. It was found that the number of wounds, blood specimens, oxygenators, and cardiotomy reservoirs contaminated with Staphylococcus aureus was related to the number of S. aureus present in the air of the OR, but that contamination with Serratia marcescens was related to the type of suction used. This form of contamination was considerably higher when air was aspirated together with blood into the suction line (p less than 0.05). The oxygenator and cardiotomy reservoir were contaminated mainly by aspirating wound fluid from the airborne-contaminated wound area. The low number of sample sites positive for S. marcescens may be due to a better preserved host defense mechanism if only wound fluid is sucked. A rather high incidence of postoperative infections occurred even in dogs operated on in an OR with a low level of airborne contamination.