Summary Three cell-free culture media (TPH, F-4, and F-7) were successfully employed to cultivate T. cruzi serially and predominantly as leishmaniforms. The simplest of these, F-7, consisted essentially of modified Tissue Culture Medium 199, chicken plasma, and chick-embryo extract. Starting with a population derived from NNN cultures that consisted of over 98% crithidiforms, the proportion of the morphologic types did not change significantly during 16 serial subcultures in TPH medium at 24.5°C. However, a larger percentage of leishmaniforms appeared in such cultures as the temperature was raised from 29.5 to 35.5°C. At the latter temperature, over 90% of the organisms became leishmaniforms after 15 passages. Thus, in TPH medium, temperature seemed to underlie the predominance of leishmaniforms. Temperature seemed to be less important than the medium itself with respect to the predominance of leishmaniforms cultured serially in F-4 medium. In this medium, regardless of incubation temperature, over 90% of the organisms became leishmaniform after several passages. Similar results were obtained in F-7 medium. The available evidence suggests that the presence of chicken plasma and chick-embryo extract was related to the predominance of leishmaniforms of T. cruzi in F-4 and F-7 media.