High endocytic activity does not result in serum sensitivity in insect stage trypanosomes. A. Phase contrast images of BSF and PCF cells taken at zero and three hours after incubation with 10% normal human serum (NHS). At zero hours both BSF and PCF parasites are intact while by three hours the BSF cells begin to exhibit vacuolation, while there is no change to the morphology of either wild type or Rab5AWT procyclic cells. Scale bar is 2 um. B. Growth curves for strain 427 procyclic line and PCF line over-expressing Rab5AQL (5AQL) cultured in different sera. The 5AQL line exhibits a rate of fluid phase and receptor-mediated endocytosis indistinguishable from bloodstream forms . Untransformed 427 procyclics and the 5AQL line were cultured in SDM-79 medium supplemented with 20% FBS, 20% NHS or 10% FBS/10% NHS for several days under normal culture conditions. Parasite density was estimated daily with a haemocytometer. While the 427 PCF and 5AQL lines demonstrate differential growth in the single sera medium, in no case is there evidence for trypanocidal activity. The probability that preferential growth is due to limiting availability of a specific factor is supported by the identical growth observed when the sera are provided as a mixture. The data are from a single experiment done in duplicate, and are representative of several similar experiments. Error bars indicate standard deviation.