In vitro culture of human peripheral blood lymphocytes (PBL) with Daudi (Burkitt lymphoma) cells results in selective proliferation of V gamma 9/V delta 2 T cells with high cytotoxicity against Daudi cells. After adoptive transfer into severe combined immunodeficient (SCID) mice, these cells exert specific anti-tumour activity against Daudi lymphoma. To test whether cytotoxic V gamma 9/V delta 2 T cells are induced in SCID mice, human PBL injected intraperitoneally were stimulated with irradiated Daudi cells (PBL/Daudi-SCID). After 7-14 days, PBL/Daudi-SCID had a significantly higher percentage of human gamma delta T cells in their peritoneal cavity, lymph nodes and blood than controls (PBL-SCID). DNA content analysis of T cell subsets from PBL/Daudi-SCID showed a significantly higher percentage of cells in S + G2 + M phases of the cell cycle in the TCR-gamma delta-1+ than in CD3+ cell population. Human cells recovered from PBL/Daudi-SCID showed specific cytotoxicity against Daudi cells. PBL/Daudi-SCID inoculated with a lethal dose of Daudi lymphoma survived significantly longer than controls. This protection was specific for Daudi cells and was not mediated by murine natural killer (NK) cells. Thus human peripheral blood T cells grafted in SCID mice proliferate in response to antigen and confer specific immunity.