IgM produced during the declining phase of the first peak of parasitaemia in rats infected with an isolate of Trypanosoma congolense was isolated on a Sephadex G 200 column equilibrated against 0.05 M Tris HCl in 0.15 M NaCl buffer. Rats passively immunised with the anti-trypanosome IgM survived infection with the same parasite isolate for an average of 6.7 days more than the controls. Apparently passive transfer of IgM can only confer partial protection against trypanosome infection.
