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SYNOPSIS. During a 7‐year period Histomonas meleagridis survived 1000 passages in Medium 199 fortified with serum and antibiotic‐killed bacteria. The histomonads were originally isolated from a chicken's cecal dropping, and the bacteria were cultivated from the cecal contents of a normal turkey. In many respects, the histomonads remained unchanged during cultivation, but in some other respects they changed considerably, perhaps irreversibly. Morphologically, the histomonads propagated in vitro showed only slight changes. When returned to birds, they resumed the structure characteristic of lumen‐dwelling individuals of this species. However, they had long since lost their ability to produce disease in either chickens or turkeys, and organisms of the tissue‐dwelling type were rarely seen. The long‐cultivated histomonads are actually as nonpathogenic as Histomonas wenrichi, but they have none of the distinguishing morphologic characteristics of the latter. As has been reported elsewhere, H. meleagridis long maintained in the tissue culture medium has also lost much of its ability to immunize either chickens or turkeys against infection with virulent strains of this parasite. Also lost in the process of adaptation to its restricted medium has been the ability to multiply satisfactorily in vitro with the complement of bacteria normal to the ceca of the birds. However, in some instances, histomonads which have become adapted to in vitro cultivation are still able to live in birds with this diversified flora. Activity of the histomonads cultivated in vitro differed but little from that of H. meleagridis freshly isolated from birds, when both were viewed under the same conditions. Histomonads from each of the above sources multiplied most satisfactorily in their accustomed habitat, probably because of a difference in nutritional requirements.

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