The aim of the current study was to determine if frozen-thawed bull semen can be treated with the acidic buffer MES (2-[N- morpholino] ethanesulfonic acid) without any detrimental effect on the motility, plasma membrane, acrosomal membrane and longevity of sperm. Frozen bull semen was obtained from a local co-operative. The semen was frozen in 0.25 mL French straws at a concentration of 80 x 106 sperm cells per millilitre. Semen of two different batches from ten bulls of four different breeds was used in this study. Three frozen semen straws of each batch were thawed at 38° C for 25 seconds. The thawed semen was pooled and then split into two aliquots. The one aliquot was used as control, whilst the other was exposed to MES treatment. The motility, plasma membrane integrity, acrosomal membrane integrity and longevity of sperm were evaluated. The effect of MES on motility was minimal as only the percentage of aberrantly motile sperm increased two hours after treatment. Although no effect on the plasma membranes were observed, it can be assumed that some damage did occur due to the fact that the acrosomal membranes were affected significantly. No significant effect was found for longevity of sperm between the control and treated samples, but a significant effect was found for both the control and treated samples over time. Although the detrimental effects caused by MES treatment would render some sperm unable to fertilise an oocyte, it is likely that a sufficient portion of sperm would survive the treatment. It is probable that this treatment would also be effective in frozen-thawed buffalo semen. The following step would be to treat semen of footand-mouth disease positive bulls with MES to establish if treatment with MES will be effective in inactivating foot-and-mouth disease virus in semen of infected bulls. Copyright / Dissertation (MSc (Veterinary Science))--University of Pretoria, 2008. / Production Animal Studies / unrestricted