The influence of cardiopulmonary bypass on the adhesiveness of blood platelets, using both bubble (Bentley and Rygg-Kyvsgaard) and membrane (Landé-Edwards) oxygenators, was investigated. With both types of oxygenators, there was a considerable fall in platelet adhesiveness during cardiopulmonary bypass, particularly with the Rygg-Kyvsgaard apparatus. The adhesiveness increased 30 minutes after bypass, but it was still not restored to the initial value.