For selective isolation of fastidious pathogenic microorganisms after addition of blood. BLOOD AGAR BASE (INFUSION AGAR) is used for isolation, cultivation of fastidious pathogenicmicroorganisms after addition of blood. This mediacan be used as general purpose mediaalso, withoutbloodaddition.Blood agar base medium can also be used for primary isolationof Haemophilusspecies, where defibrinated sheep blood is used to enrich the medium. Medium contains a rich nutrient base,which provides optimal growth conditions for all relevant microorganisms. Tryptone and Brain Heartinfusion suppliesamino acids, minerals and other essential growth factors in the medium for the growthof microorganisms. Sodium chloride maintains the osmotic balance of the medium. Agar is a solidifyingagent. On addition of blood the pH value of 6.8 stabilizes and favours the formation of clear hemolysiszones. Fresh, defibrinated sheep blood is most suitable for determent hemolysis forms. It gives best results for Group A Streptococci. But sheep blood fails to supportgrowth of Haemophilus haemolyticusdueto deficiencyofpyridine nucleotides. However,addition ofhorse blood supports H. haemolyticusgrowth and haemolysis and the growing coloniesmimic Streptococcus pyogenes.