Although gamma delta T cells are found in increased numbers in the spleens of humans and mice infected with malaria, it is not known if these cells are necessary components of an effective immune response. The surface phenotype of spleen cells obtained from mice infected with avirulent Plasmodium chabaudi adami or virulent Plasmodium chabaudi chabaudi were examined using anti-delta or anti-alpha beta T-cell-specific reagents and flow cytometry. Levels of parasitaemia, red blood cell (RBC) counts, and survival times were followed in mice depleted of tumour necrosis factor (TCR)gamma delta+ or TCR alpha beta+ T cells. Numbers of gamma delta T cells increased in the spleens of control antibody-treated infected mice, but not in mice depleted of TCR gamma delta+ or TCR alpha beta+ T cells. Mice depleted of gamma delta T cells had levels of parasitaemia, RBCs, and survival rates similar to control antibody-treated mice. However, mice depleted of TCR alpha beta+ T cells had higher levels of parasitaemia, lower RBC counts, and decreased survival rates. These results indicate that TCR alpha beta+ but not TCR gamma delta+ T cells play an essential role in host defense against P. chabaudi infection in mice.