Mice from Biozzi high (H) and low (L) antibody responder lines were infected either with non-lethal Plasmodium yoelii 17X, with a lethal variant of the same parasite or with P. chabaudi (non-lethal). H mice recovered from infection with both P. yoelii and its normally lethal variant, whereas L mice died from both. However, L mice showed better initial control of parasitaemia with all three infections. Both H and L mice infected with P. chabaudi showed a similar rapid reduction of parasitaemia once it had reached 40-50%, but L mice were unable to clear the infection completely. When infected with either non-lethal P. yoelii or P. chabaudi, H mice made substantially more antibody than L mice, especially around the time of recovery. The courses of parasitaemia suggest a role for non-antibody factors in early control of infection and for antibody in recovery from P. yoelii. Although antibody was apparently necessary for complete recovery from P. chabaudi, the first rapid fall in parasitaemia appeared to be unrelated to antibody levels.