The effects of 5-aminolevulinic acid (ALA)-mediated photodynamic therapy (PDT) and gamma radiation, and PDT and hyperthermia (HT) are investigated in human glioma spheroids. In the case of ionizing radiation and PDT, the degree of interaction between the two modalities depends in a complex manner on a number of factors, including light fluence, fluence rate and radiation dose. It is shown that, under certain conditions, PDT and gamma radiation interact synergistically to produce enhanced cell kill. The degree of interaction appears to be independent of both sequence and time intervals investigated. TUNEL assays show that low fluence-rate ALA-PDT is a very efficient inducer of apoptosis, whereas neither high-fluence rate PDT nor ionizing radiation results in significant apoptosis in this system. Synergistic interactions between HT and PDT are observed when the two modalities are given concurrently. The degree of synergism increases with increasing temperature and light fluence. It is shown that apoptosis is the primary mode of cell death following combined HT and low-fluence rate ALA-PDT.