A new hollow fiber membrane oxygenator, the Medtronic Maxima Forte, was tested for gas transfer, blood path resistance and blood handling characteristics in a standardized setting with surviving animals. Three calves (mean body weight: 71 +/- 9.6 kg) were placed on cardiopulmonary bypass at a mean flow rate of 50 ml/kg/min for six hours. The circuit included the Maxima Forte oxygenator. The animals were weaned from cardiopulmonary bypass and then from the ventilator. After seven days, the animals were sacrificed electively. Physiologic blood gas values could be maintained throughout perfusion in all animals. Mean pressure drop through the oxygenator varied between 49 mmHg and 66 mmHg. The respective baseline values for red blood cell count, white blood cell count and platelets were 8.90 +/- 1.26 10(6)/mm3, 7.46 +/- 3.17 10(3)/mm3. and 680 +/- 216 10(3)/mm3. Red blood cell and platelet counts dropped slightly to 7.26 +/- 1.61 10(6)/mm3 and 400 +/- 126 10(3)/mm3 at the end of the bypass, whereas the white blood cell count increased up to 9.13 +/- 5.25 10(3)/mm3. All three cell lines returned to near their baseline values after seven days. Blood trauma evaluated as a function of plasma hemoglobin (plasma Hb) and lactate dehydrogenase (LDH) showed stable values during all the perfusion time. Both peaked at 24 hours before returning to their baseline values at seven days. LDH showed a statistically significant variation: 3255 +/- 693 IU at 24 hours versus 2029 +/- 287 IU at baseline (p = 0.04). The variation of plasma Hb was not statistically significant (93.5 +/- 7.7 mumol/l at 24 hours versus 77.3 +/- 52.3 mumol/l at baseline) indicating a weak effect of the perfusion on blood trauma. The Medtronic Maxima Forte hollow fiber membrane oxygenator offered good gas exchange capabilities, a low pressure drop, and low blood trauma over a prolonged perfusion time of six hours in this evaluation.