Army Medical Department. MATERIALS AND METHODS

Cultures of Entamoeba histolytica were subjected to pure gamma radiation from a cobalt-60 source. No effect upon population growth was noticed following dosages below 100,000 RAD. A noticeable increase in lag phase was observed following ex-posure to about 100,000 RAD. A dose of more than 195,300 RAD resulted in death as indicated by failure of cultures to grow. The relation between increasing lag phase and increasing dosages of radiation is represented by a logarithmic straight line. Because of the paucity of standardized methods for experimenting with the entozoic amebae, information about their response to radiation has been scanty. Nasset and Ko-foid (1928) incubated radium needles with cultures of Entamoeba histolytica (their Enda-moeba dysenteriae) and recorded stimulation of in vitro growth. The radiation was com-posed, for practical purposes, of gamma rays but the level of intensity of ionization must have been comparatively low, for continuous exposures over 12-hour intervals failed to affect the rate of division adversely. The single other reference to the radio-sensitivity of parasit.ic amebae is t’he work of Sadun et al. (1950), who found that 120,000 roentgens of X-rays failed to inactivate com-pletely amebae growing in Shaffer-Frye or Balamuth media. The normal cycle of popu-lation growth was not visibly affected by exposures up to 30,000 roentgens, but ex-posures of 60,000 and 120,000 roentgens seemed to inhibit growth. In the following study an attempt has been made to amplify the work of Sadun et al. and to identify t’he level of ionizing radiation which is lethal for E. histolytica. The radia-tion consisted of pure gamma rays and dosages are reported in terms of RAD (cor-responding to 100 ergs per gram)