Abstract 1. 1. The toxicity of emetine to one simian and four human cultures of Entamoeba histolytica in vitro was assayed on eight occasions over a period of 4 1 2 months. The inhibitory concentration for any one culture varied from experiment to experiment, and there was an overall difference between all cultures. The culture of simian origin showed the greatest resistance to the drug. 2. 2. No increase in resistance to emetine was observed when one culture of E. histolytica was maintained by subinoculation in sublethal concentrations of the drug over a period of five months. 3. 3. The response to emetine therapy of infections induced in rats by two cultures of E. histolytica was measured on several occasions over a period of 19 months. The doses of emetine necessary to produce a standard therapeutic effect were different for the two cultures. 4. 4. Repeated passage in rats of an amoebic infection exposed to subtherapeutic doses of emetine did not materially alter the response of the infection to the drug.