To elucidate the effects of bubble and membrane oxygenators on platelet integrity, we developed a quantitative method of determining platelet lysis during cardiopulmonary bypass. Two groups of dogs whose platelets had been labeled with indium 111 were subjected to 1 hour of cardiopulmonary bypass. In Group A (bubble oxygenator), platelet lysis as measured by free plasma 111In levels increased from 6% ± 1% to 33% ± 7% during bypass. In Group B (membrane oxygenator), plasma 111In levels increased from 5% ± 2% to 10% ± 6% during bypass (p < 0.01). After 1 hour of bypass, the ratio of 111In-labeled platelets to prebypass levels was 36% ± 8% in Group A and 67% ± 9% in Group B. Platelet deposition on the oxygenator was greater in bubble oxygenators (19% ± 4% of total injected 111In). These data indicate that membrane oxygenators maintain a higher circulating platelet count both intraoperatively and postoperatively and result in less platelet destruction than bubble oxygenators following 1 hour of cardiopulmonary bypass in dogs.