MATERIALS AND METHODS

Experiments were carried out with LACA mice infected with B. microti (King's strain) and B. hylomysci (Antwerp strain) which had been used for other purposes and which had recovered, ie 35 days after infection. Thirty-seven B. microti mice and 19 B. hylomysci mice were used. Tail blood smears were prepared daily and examined to detect any spontaneous recrudescences. 0· 5 ml of blood was obtained weekly from each animal (by rupturing the retro-orbital plexus) and diluted with'3 ml of Alsever's solution. One ml of this was use!! to inoculate intraperitoneally each of three splenectomized mice. Blood smears were prepared from the inoculated mice daily for two weeks and examined for parasitaemia. If no infection was detected in the three mice the donor was then splenectomized and blood smears examined for a further two weeks. If no infection v.'as then detected, the donor was challenged by the intraperitoneal injection of 108 mouse erythrocytes infected with the homologous species of parasite and with the other parasite two weeks after the first challenge. Blood smears were examined daily for two weeks following each challenge.