Ethylhydrocupreine hydrochloride, used for the differentiation of true pneumococci (Streptococcus pneumoniae) from other alpha- haemolytic streptococci. Filter paper discs 5 -8 mm in diameter, impregnated with a 1/4000 solution, are placed on blood agar plates inoculated with the suspect strains. Only pneumococci are inhibited, a 5 -10 mm zone of inhibition surrounding the disc.