The transfer of spleen cells from (BALB/c x C57Bl/6) F1 mice recovered from a Plasmodium chabaudi chabaudi AS infection into irradiated syngeneic recipients conferred protection. Neither elimination of Thy-1+ cells nor in vitro irradiation of immune cells before transfer affected protection while both anti-Thy-1 treatment and irradiation abolished the appearance of anti-P. c. chabaudi antibodies in the recipients. Superinfection of immune spleen cell donors did not improve their capability to transfer protection which was also unaffected by anti-Thy-1 treatment. The serum of mice after one infection was only marginally protective when transferred into irradiated recipients and a second infection improved the protective activity of serum which was not further improved by six infections. The co-transfer of immune serum and immune cells did not result in any synergistic effect. On the other hand, when P. c. chabaudi AS (BALB/c x C57Bl/6) F1 infected mice were challenged with a high dose of Plasmodium yoelii 17XL at crisis, the mice were unable to control the heterologous parasite. When mice were challenged with P. yoelii 17XL several weeks after infection with P. c. chabaudi AS, a good degree of cross-protection was observed.