Morphogenesis of blood stream trypomastigotes in the cell free culture medium F69 at 37 degrees C for 10 days showed qualitative differences either with or without human plasma. Without human plasma, blood stream trypomastigotes performed only one cycle before disappearing and the culture kept growing as amastigotes and epimastigotes until the end of the experiment. In contrast, human plasma induced multiple cycles of transformation. The sequence was blood stream trypomastigotes, regressive parasites, amastigotes, progressive parasite and again trypomastigotes. Human plasma preserved the trypomastigote stage, produced a blockade of the epimastigote stage and inhibited the division of amastigotes. In this experimental model, human plasma modified the biological cycle of T. cruzi by inducing or inhibiting different stages.