Interferon and retinoic acid are active agents for the treatment of cervical cancer, but their mechanisms of action are unclear. Results of [3H]thymidine uptake assays showed that exposure to pharmacologic concentrations of interferon-α (IFN-α) and all-trans-retinoic acid (RA) for 72 hr inhibited growth of the cervical cancer cell lines ME-180, 283, SiHa, C33-A, 621, CaSki, HeLa, and B132. CaSki and SiHa cells continuously exposed to IFN-α or RA or both for 9 days developed resistance to growth inhibition, and growth resumed at a rate comparable to control after removal of agents. Similar assays showed no significant difference in effects of RA and its cis isomer. Assays for lactate dehydrogenase release revealed no significant lysis of any cell line following exposure to IFN-α, RA, or their combination. In organotypic culture, cells grew in a pattern histologically similar to carcinoma in situ, and exposure to IFN-α and RA for 14 days yielded no change in this pattern. Immunohistochemical analysis showed no change in cytokeratin expression by cells in organotypic or monolayer culture. The major in vitro effect of IFN-α and RA on cervical cancer cell lines appears to be reversible inhibition of proliferation.