Susceptible African trypanosomes are lysed by a factor in human serum (HS), which presumably binds to their surface and is then internalized. It has been suggested that internalization of the factor is required for lysis. The hypothesis predicted that if the trypanolytic factor (TLF) binds and is endocytosed by trypanosomes, the lytic activity in HS should be removed by them. The experiments in this report have demonstrated that the lytic activity in HS can be almost completely removed. This was shown using both human serum sensitive (HSS) and resistant clones. As it might have been expected, HSS cells remove a greater percentage of the trypanolytic activity. In addition, the hypothesis also predicts that if the TLF is processed and activated from inside the trypanosome, its activity should be detected in the lysates of thoroughly washed trypanosomes previously incubated with HS. The results showed that the lysates consistently contained a soluble active form of the TLF that has been internalized by the trypanosomes. Antiserum specific to human high-density lipoprotein was found to neutralize the trypanolytic activity present in the lysates but failed to prevent the lysis of trypanosomes already exposed to HS.