The goal of the study was to investigate the effect of antioxidant supplementation on the quality of frozen-thawed flow cytometrically sorted bull spermatozoa. Twelve ejaculates from two Holstein Friesian bulls were sorted according to the Beltsville Sperm Sexing Technology. Each ejaculate was divided into three parts and processed as (i) unsorted controls, (ii) according to a standard sorting protocol and (iii) in the presence of different antioxidants (S-AO). Cooling and freezing of the samples were performed in the same way for all three groups, except that antioxidants were added to the TRIS-egg-yolk freezing extender for those semen samples that were already sorted in the presence of antioxidants. The semen quality in frozen-thawed samples was determined by morphology analysis immediately after thawing, motility estimation in a thermo-resistance test after 0, 6, 12 and 24 h incubation at 37 degrees C and Fluorescein isothiocyanate conjugated PNA/propidium iodide (FITC-PNA/PI) staining after 0, 12 and 24 h of incubation at 37 degrees C. There was a significantly higher (p < 0.05) percentage of motile spermatozoa in S-AO samples in comparison to unsorted frozen-thawed control at 0, 6 and 24 h after thawing and compared with normally sorted samples at all times after thawing. The percentage of damaged acrosomes was significantly lower (p < 0.05) in S-AO samples than in the unsorted controls (20.8 +/- 6.9% vs 30.3 +/- 12.0%). The percentage of morphologically abnormal spermatozoa in this group was significantly lower (p < 0.05) than in the unsorted controls and normally sorted samples (25.8 +/- 5.2%, 36.0 +/- 12.5% and 35.1 +/- 7.4%, respectively). Analysis of frozen-thawed spermatozoa with FITC/PI revealed no significant difference in membrane integrity at 0 and 12 h after sorting, but after 24 h of incubation the S-AO samples had a significantly higher (p < 0.001) percentage of spermatozoa with intact membranes in comparison to unsorted controls and normally sorted semen (40.7 +/- 6.3%, 7.8 +/- 4.7% and 7.4 +/- 4.6%, respectively). The percentage of acrosome-reacted spermatozoa was significantly lower (p < 0.05) in the S-AO samples than in the unsorted controls (14.1 +/- 7.5%, 23.4 +/- 5.4% and 28.8 +/- 6.3% vs 25.9 +/- 14.4%, 38.5 +/- 16.7% and 79.8 +/- 4.1%, for 0, 12 and 24 h after thawing, respectively) and in comparison to normally sorted semen 24 h after thawing (67.3 +/- 10.0%). This study demonstrates the highly protective effects of antioxidants on the quality of flow cytometrically sorted frozen-thawed bull spermatozoa.