Costimulation plays very important role in T cells activation and tumor immunity. B7-1 is alone of the major costimulatory molecules both in human and rodents. Previous work indicated that B7-1 gene transfected EL-4 lymphoma can induce antitumor immunity in vivo. This paper showed that inoculation of EL-4B7-1⁺ plus IL-2 could elicit an antitumor effects in vivo and in vitro. Immunization with EL-4B7-1⁺ tumor cells or EL-4B7-1⁺ tumor cells plus IL-2 could significantly prolong the survival time of the EL-4 tumorbearing mice and also delay the occurrence of the tumor node in vivo. A strong proliferation response and CTL activity as well as the increased IL-2 and TNF production of the splenocyte from the immunized mice with EL-4B7-1⁺ or EL-4B7-1⁺ plus IL-2 were seen in the in vitro TLMC system. These finding indicated that the costimulatory molecule is necessary for inducing an effective antitumor immunity and IL-2 optimal production.