OBJECTIVE Oxidative damage and inflammation are believed to play an important role in postoperative complications after cardiopulmonary bypass. During bypass, a prime solution with a high antioxidant capacity may reduce the oxidative damage and inflammation. We investigated total antioxidant capacity and individual scavengers during the preparation of 2 different prime solutions. METHODS The prime solutions were prepared with either pasteurized human albumin or fresh frozen plasma. The total antioxidant capacity was measured with the total radical antioxidant parameter assay and with the ferric-reducing ability of plasma assay. The individual scavengers vitamin C, sulfhydryl groups, uric acid, and total protein were measured before, during, and after the prime preparation. Malondialdehyde was measured as a parameter for lipid peroxidation. RESULTS Neither prime solution showed a total radical antioxidant parameter value. The ferric-reducing ability of plasma value of prime solutions was lower than that of undiluted human albumin or fresh frozen plasma. Addition of mannitol did not increase the ferric-reducing ability of plasma value. Vitamin C was only found in the fresh frozen plasma prime. Both prime solutions contained sulfhydryl groups and uric acid in low concentrations. During ultrafiltration, low-molecular-weight antioxidants were lost into the ultrafiltrate. CONCLUSIONS We showed that prime solutions based on either albumin or fresh frozen plasma had very low antioxidant capacity and that ultrafiltration of the prime solution further lowers this capacity. A prime solution with a low antioxidant capacity may increase oxidative stress in neonates undergoing cardiopulmonary bypass.