An association has been demonstrated between intravascular microemboli and organ injury during cardiopulmonary bypass (CPB). Air may be inadvertently introduced into the venous line during CPB resulting in the formation of gaseous microemboli (GME). We studied the ability of CPB circuits, from five different manufacturers, to remove GME originating from the introduction of air into the venous line. Using an in vitro model of adult CPB, 60 ml of air was introduced into the venous line and the progression of GME through the circuit components was monitored at 5 locations. In all circuits GME were detected in the arterial line following the introduction of air into the venous line. There was a wide variation between manufacturers in the ability of the circuit to remove GME. Air introduced into the venous line during CPB results in the formation of GME that are able to pass through all the circuit components including the arterial filter. The quantity of GME detected in the arterial line is influenced by the design of the circuit components and varies between manufacturers. Air in the venous line should be avoided and if present it must be dealt with promptly.