Y strain parasites (Wellcome stock) were cloned as epimastigotes by limiting dilution and one clone, Wel Tryp A2 was used to infect cell cultures and irradiated mice. The fibrosarcoma line, M4, was susceptible to infection with Wel Tryp A2 trypomastigotes but, after the first round of intracellular infection, proliferation and differentiation, failed to support parasite growth in long-term continuous culture. In contrast, a muscle-derived cell line, S2, produced regular waves of extracellular trypomastigotes and amastigotes during 80 days of continuous culture reported here. These infected S2 cultures have remained in continuous culture for up to 18 months over a 5-year period and have yielded in excess of 108 parasites/20 ml culture at 4–6 day intervals. Extracellular amastigotes produced in these cultures have a close morphological, immunological and biochemical analogy to intracellular parasites derived from rodents, and have maintained an absolute dependence on host cells for their growth and proliferation. These amastigotes produced low-grade chronic infections in normal CBA/T6 mice but produced acute, fatal infections with high parasitaemia in mice of the same inbred strain given 900 rad. whole-body irradiation prior to infection. Although present findings relate to a single parasite clone, the parent stock from which the clone was derived showed similar growth characteristics when co-cultured with S2 cells.