In recent years, atabrine (dihydrochloride of methoxychlorodiethyl amino-pentyl amino-acridine) has been described as a specific parasiticide for Giardia lamblia. Our clinical experience has substantiated this claim, and we are of the opinion that it is now possible to study the effects of giardiasis on human beings much more accurately than it was before a specific remedy for this parasite was known. Our interest in the subject was stimulated by a meeting of the Pan American Medical Association in 1938, and it was at that time that we first began our investigations. Our purpose in this paper is to review briefly a few pertinent contributions to our knowledge of the subject and to present an analysis of the symptoms in 100 cases of giardiasis and the effect of treatment with atabrine in 46 cases. LITERATURE Giardia lamblia, or Giardia intestinalis, is a flagellate of the intestine. Geographically, it has a wide