Rat 6 cells are not transformed by treatment with the well-known carcinogens benzo[a]pyrene (BP) or N-methyl-N-nitro-N'-nitrosoguanidine (MNNG). Upon retroviral transduction of the mouse c-myc gene, Rat 6 cells showed mildly altered morphology and formed microcolonies in soft agar; furthermore, they could be transformed by BP and MNNG to form large colonies in agar (Hsiao et al. (1992) Mol. Carcinogenesis, 5, 140-154). In the current report, we tested the sensitivity of the c-myc-overexpressing cells (Rat 6/c-myc) to two additional chemicals: 5-azacytidine and MnSO4. These chemicals differ from the direct-acting mutagens tested previously. 5-Azacytidine, a potent DNA methylation inhibitor, induced growth of large colonies in soft agar cultures of Rat 6 or Rat 6/c-myc cells. On the other hand, MnSO4 only induced transformation in Rat 6/c-myc cells, but not the parental Rat 6 cells. Transformants induced by 5-azacytidine lost c-myc-induced apoptotic cell death, whereas MnSO4-induced transformants showed a higher degree of apoptosis than the parental Rat 6/c-myc cells. These results suggest that MnSO4 co-operates with overexpressed c-myc in inducing transformation, while 5-azacytidine transformation is independent of c-myc overexpression and may involve alterations in the regulation of apoptosis.