Butylated hydroxytoluene (BHT) has proven to be efficient as a supplement for cryopreservation boar spermatozoa (Roca et al. 2004 J. Androl. 25, 397–405). Moreover, it has been successfully used as an egg yolk substitute to cryopreserve goat spermatozoa (Khalifa and El-Saidy 2006 Anim. Reprod. Sci. 93, 303–315). The objective of this study was to evaluate the effectiveness of BHT as an egg yolk substitute for freezing boar spermatozoa. Nine sperm-rich ejaculate fractions were collected from 3 boars (3 ejaculates per boar) using the gloved-hand method. After centrifugation (2400g for 3 min), the sperm pellet of each ejaculate was split into 5 aliquots. The aliquots were diluted (to a final concentration of 1 � 109 sperm/mL) in a Tris-citric-glucose extender with 3% glycerol and supplemented with 20% egg yolk (positive control, PC aliquot) or BHT at the final concentrations of 0 (negative control, NC aliquot), 0.2, 0.4, and 0.8 mM. Diluted semen samples were dispensed into 0.5-mL straws, and frozen in a programmable cell freezer at 20�C min. Thawing was carried out in a water bath at 70�C for 8 s. Post-thaw sperm survival was assessed according to total sperm motility (TSM, %) using a CASA system (SCA�; Microptic, Barcelona, Spain), and plasma membrane integrity (PMI, %) and acrosome membrane integrity (AMI, %) using a flow cytometric procedure (SYBR-14/propidioum iodide/FITC-phycoerythrin), at 30 and 150 min post-thawing in diluted Beltsville thawing solution with spermatozoa held in a waterbath at 37�C (3 straws per ejaculate). Data were analyzed using a ANOVA mixed model including the main effects of aliquot, boar, post-thaw assessment time, and their interactions, with ejaculate and straw as random effects. All main effects had significant influence (P ≤ 0.01) in all post-thaw sperm assessments. However, no interactions (P ≥ 0.05) among main effects were shown. Data were combined for the 2 post-thaw assessment times. The best (P ≤ 0.05) post-thaw sperm quality (mean � SEM) was achieved in PC aliquots (47.11 � 3.10, 58.98 � 2.78, and 51.35 � 3.42 for TSM, PMI, and AMI, respectively). In NC aliquots, the percentage of TSM, PMI, and AMI were always below 1% (P ≤ 0.05). BHT has a beneficial (P ≤ 0.05) effect on post-thaw sperm assessments, and no differences (P ≥ 0.05) among concentrations were shown. The mean post-thaw sperm quality in the BHT aliquots was 8.50 � 0.80, 20.29 � 0.53, and 16.03 � 0.55 for TSM, PMI, and AMI, respectively. On the basis of these data, we can conclude that BHT has a protective effect for boar spermatozoa during the cryopreservation process. However, BHT alone is insufficient to replace the protective effect of egg yolk. This work was supported by CICYT (AGF2005-00706), Madrid, Spain.