OBJECTIVE The effects of the heat shock protein 90 (Hsp90) inhibitor geldanamycin (GA) were examined on the radiosensitivity and signal transduction pathways in human tumour cell lines. METHODS Two human cell lines, SQ-5 and DLD-1, derived from lung carcinoma and colon adenocarcinoma, respectively, were incubated for 16 h at 37 degrees C in medium containing 0.2 microM GA. The cells were then irradiated with X-rays and incubated with GA for a further 8 h. Radiation sensitivity was determined by clonogenic assays and protein levels were examined by Western blotting. RESULTS GA radiosensitized both cell lines, but potentiated X-ray sensitivity more in SQ-5 than in DLD-1 cells. It was found that GA depleted EGFR and ErbB-2 in DLD-1 cells and depleted only ErbB-2 in SQ-5 cells. GA also reduced the expression of Akt and phosphorylated Akt (pAkt) expression in SQ-5 cells. In addition, the ratio (%) of apoptotic cells and poly [ADP-ribose] polymerase cleavage increased in SQ-5 but not in DLD-1 cells after exposure to GA and X-ray irradiation. The findings suggest that GA enhances the radiation sensitivity of human tumour cells by inhibiting the EGFR signal transduction system and the Akt signalling pathway. CONCLUSIONS Targeting Hsp90 with GA provides a promising experimental strategy for radiosensitization of carcinoma.