Abstract
In the chemotherapy of trypanosomiasis of man and animals, it is possible to produce arsenic-fast strains of various trypanosomes after these organisms have been subjected a few times to the action of certain arsenic compounds short of the sterilizing dose. Experiments of this nature have been conducted with atoxyl, arsacetin and other arsenic compounds. If the organisms are not all destroyed after the initial injection, the offspring of the remaining ones are more resistant to the action of the drug than the preceding generation. A larger dose of the arsenical compound is required to destroy the organism and to overcome this increased resistance. This increased resistance may be developed to such an extent that the amount of the arsenic compound required to exert a parasitotropic action is in excess of its organotropic action. Indeed, the production of drug-fast strains of various trypanosomes is an admitted failure of chemotherapy of trypanosomiasis

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