These discs are recommended for use with pure cultures in the presumptive identification of pneumococci.

The 6mm dia. (¹/₄") discs are made from high quality absorbent paper that has been impregnated with approximately 5µg ethylhydrocupreine hydrochloride (optochin), which markedly inhibits the growth of pneumococci, but not of other streptococci. Pneumococci can, therefore, be differentiated from other alpha-hemolytic streptococci by the formation of a zone of inhibition around a disc placed on a blood agar plate heavily inoculated with a pure culture suspected to be Streptococcus pneumoniae.