Viability of rat 3Y1 fibroblasts transformed by adenovirus type 12 (Ad12) was markedly impaired by the administration of dilinoleoylglycerol (DLG) to the culture medium. To identify the gene(s) of Ad12 responsible for the high sensitivity to DLG, we established several transformed sublines of 3Y1 induced by the viral E1A gene or by the mutants of Ad12 which have mutations in the E1A region. All of the transformed sublines of 3Y1 expressing either the 12S or the 13S message from the E1A region were highly sensitive to the cytotoxicity of DLG. We propose that the high sensitivity of Ad12-transformed cells to the DLG mediated cytotoxicity is attributable to the common function of E1A-12S and E1A-13S mRNA products.