Objective. Glutamine is proposed to protect bowel from radiation. However, glutamine may decrease cancer's radiosensitivity. We evaluate glutamine's effect on the growth rate and radiosensitivity of two cervical carcinoma cell lines in vitro. Methods. HeLa and CaSki cells were seeded at 3000 cells/well in glutamine-free medium. An increasing amount of glutamine (0.4, 10, and 20 mM) was added to the respective plates, incubated, and irradiated with a single fraction of 0.5, 1, 3, and 6 Gy. Using a growth inhibition assay and photometric analysis, the viable cells were counted on day 8. Cell counts represent a mean ± standard deviation from six experiments and are expressed in 103 cells. Analysis of variance was performed. Results. In nonirradiated HeLa plates, absence of glutamine results in 5.7 ± 1.2 cells/well. Addition of glutamine at 0.4, 10, and 20 mM to nonirradiated cells significantly (P < 0.0001) increased growth to 79.1 ± 10.0, 122.5 ± 9.0, and 114.3 ± 13.9 cells/well, respectively. In culture plates irradiated with 6 Gy, HeLa cells supplemented with 0.4, 10, and 20 mM of glutamine showed lower cell counts (P < 0.008). A similar significant growth suppression at 6 Gy in comparison to 0.5, 1, and 3 Gy was observed (P < 0.01). CaSki cells showed similar patterns. Conclusions. Growth of HeLa and CaSki cells in vitro requires a minimum of 0.4 mM of glutamine in the medium. Supraphysiologic glutamine concentration does not increase tumor growth or radioresistance. Glutamine should be evaluated further as a potential bowel radioprotector.