Optochin Discs are used for identification and differentiation of Streptococcus pneumoniae and Viridans Streptococci.Alpha haemolytic (viridans) streptococci and Pneumococcus (Streptococcus pneumoniae) cannot be easily distinguished on Blood Agar plates as pneumococci strain shows partial clearing of blood and greenish discolouration (a-hemolysis). Optochin is inhibitory for pneumococcal growth whereas other streptococci strains show good growth or a very small zone of inhibition.