Cysts of E. histolytica, freed from bacteria by micro-isolation, were introduced into several kinds of egg media, broth medium, and into Locke''s solution, which were dispensed in microculture tubes constructed from capillary glass tubing approximately 1 1/2 mm. in diameter. From 5 to 25 cysts were used per microculture. The microculture tubes were then sealed in a microflame, incubated at 37[degree]C and examined at intervals under water through the compound microscope for evidence of excystation. From 800 microcultures in egg and broth media, containing a total of approximately 7,000 cysts, the rates of excystation varied from less than 1% to 17%. The rates of excystation in microcultures with organism t were only slightly higher than in the bacteria-free cultures. Examination of samples of the various lots of cysts showed from 25 to 75% with 4 nuclei and indicated that the rates of excystation were probably below optimum in all cases. E. coli excysted under the same experimental conditions and at approximately the same rates as E. histolytica.