The costimulatory signal through CD80 or CD86 to its counterreceptor CD28 or CTLA-4 on T cells has been shown to play an important role in the induction of T-cell-mediated immunity against tumors. In the present study, we examined the expression of CD80 and CD86 in the cell lines derived from human gastric, esophageal and colorectal carcinomas at the mRNA level, by means of a reverse transcriptase/polymerase chain reaction analysis and, for their surface expression, using a flow-cytometric analysis with monoclonal antibodies (mAb). The expression of mRNA for CD80 or CD86 was detected in all 18 cell lines tested, except for CD86 on one cell line. The cells from 13 (72%) or 12 (67%) of these cell lines expressed the surface CD80 or CD86 molecule, detected with the respective mAb. The surface expression of CD80 or CD86 was increased in four to five of the six cell lines tested after a culture with interferon gamma (IFN gamma). In addition, the up-regulation of CD80 or CD86 expression by IFN gamma was inhibited by interleukin-10 (IL-10). These results indicated that, in the cell lines derived from human gastrointestinal carcinoma, both the costimulatory molecules CD80 and CD86 were detectable in the majority of the cell lines examined at the mRNA and protein levels, and could be regulated with the cytokine IFN gamma or IL-10.