Lysed horse blood is used for special purposes in culture media. It has been used for many years in Corynebacterium diphtheriae media, where better growth was observed after lysis of the horse blood by the tellurite in the medium. It is also documented that lysed blood stimulates the growth of Haemophilus influenzae due to the release of Nicotinamide Adenine Dinucleotide in the horse blood from the ruptured erythrocytes. In antibiotic susceptibility testing, lysed horse blood is added to the medium to improve the reactions with trimethoprim and sulphonamides. Most culture media, unless specially processed for susceptibility testing, contain amounts of thymidine which can antagonise the inhibitory effects of these antimicrobials. When horse blood is lysed the erythrocytes release an enzyme thymidine phosphorylase which converts thymidine into the much less antagonistic compound thymine.