Tumor formation was examined in mice in which the beta -chain gene of alpha betaT was knocked out (alpha betaT-KO mice) or the delta -chain gene of gamma deltaT was knocked out (gamma deltaT-KO mice). Development of Hepa 1-6 cell hepatoma was observed in six of six alpha betaT-KO mice, in four of six wild-type mice. and in only one of six gamma deltaT-KO mice. These results imply that gamma deltaT cells play a role in suppression of killer cell activity in tumor-bearing mice. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.