This paper describes the in vitro infection of human established fibroblast lines by Leishmania mexicana amazonensis amastigotes. Intracellular parasites were located within vacuoles. The proportion of infected cells reached a peak of about 50% on Days 2 or 3, and decreased to almost 0 on Days 6 or 8. Transmission electron microscopy was used to document different stages of the degeneration of L. m. amazonensis amastigotes on Days 4 and 5. The proportion of infected fibroblasts decreased in both the control and the irradiated cells, indicating that dilution of the parasites by cell multiplication was not the main factor in the observed decrease of the infection with time. This conclusion was also supported by studies with fibroblasts prelabelled with 3H thymidine. In these studies, total cell associated radioactivity as well as the total and TCA precipitable radioactivity of the culture medium were similar for infected and non-infected fibroblasts during the course of infection. The abortive infection of fibroblasts by L. m. amazonensis provides a potentially useful model for the study of the relationship between host cells and intracellular parasites.