In order to study the potential role of gamma delta T lymphocytes in cytotoxicity against breast carcinoma cells, normal peripheral blood gamma delta cells were isolated and triggered with an alloantigenic lymphoblastoid B cell line and recombinant interleukin-2 and cloned. The clones expressed a CD3+V gamma 9+V delta 2+CD4-CD8- (or CD8+) phenotype. Five clones were cytotoxic to the Molt 4 T cell leukemia, but not to the alloantigen, whereas one clone lysed the alloantigen, but not the Molt 4 line. Clones that were cytotoxic to Molt 4 were either spontaneously cytotoxic against MCF 7 breast carcinoma cells or could be induced to kill MCF 7 cells by monoclonal antibodies specific for the gamma delta T cell receptor. In situ staining demonstrated that V delta 2+ as well as other subsets of gamma delta T cells can be detected in the lymphocytic infiltrate within breast carcinomas. These data showing that V delta 2+ T cells can recognize and kill cells of breast carcinoma lineage in vitro, and that cells expressing V delta 2 genes in their T cell receptor structure can be detected in the tumor, suggest that further studies of the nature of the interactions between V delta 2 T cells and breast carcinoma cells are warranted.