The tumoricidal effect of interstitial photodynamic therapy (IPDT) using Photofrin was found to increase when combined with the bioreductive alkylating agent mitomycin C (MMC) and, to a lesser extent, with the indoloquinone EO9. When MMC was given prior to IPDT or RIF1 tumours, the light dose required for a given regrowth time or for 50% cure was reduced by a factor of 2 compared with IPDT alone. MMC given immediately after illumination did not increase the effects of IPDT, although MMC plus illumination without photosensitizer produced a significant increase in regrowth time compared with MMC or light alone. Combination of IPDT with EO9, given directly before illumination, only marginally increased the tumour regrowth times at non-toxic doses. These results demonstrate that combining IPDT with MMC greatly improves the tumour response. Factors such as PDT-induced hypoxia, pH changes, temperature increases and production of toxic reactive oxygen species by both drugs may play a role in the enhanced MMC cytotoxicity.