All attempts to identify Trypanosoma (Trypanozoon) brucei species trypanosome strains by their sensitivity or resistance to human serum in vitro have so far been unsuccessful. Wet-film examination of T.(T.) b. brucei, T. (T.)b. rhodesiense and T. (Nannomonas) congolense trypanosome strains, both before and after in vitro incubation in fresh human blood for 5 hours at 37 degrees C showed that serum-sensitivity or serum-resistance is not a constant and reproducible property of a trypanosome strain and is totally unrelated to infectivity.