To prepare 250 ml. of medium, 36 Gm. of dehydrated egg yolk are mixed with an equal amount of dist. water, and then added to 125 ml. of 0.8% NaCl soln. This mixture is boiled directly for 10 min., or heated in a covered double-boiler for 20 min. after the temp. of the infusion reaches 80[degree] C. The infusion is filtered, auto-claved, and then mixed with an equal vol. of M/15 K phosphate buffer adjusted to pH 7.5. This medium is then dispensed in tall tubes, autoclaved, and stored until use. Particu-late rice starch is added just before use. Wilson liver concentrate 1-20 in conc. of 0.5% may be added for enrichment. Data are presented to indicate the effects of varying the conditions of cultivation, including such factors as fractions of whole egg employed, concns. of egg yolk, osmotic pressure and salt concn., adjustment of the buffer, and aging of the medium. E. histolytica shows considerable tolerance toward variation in these factors, within the range observed in preparing this culture medium.