delta-Aminolevulinic acid (ALA) causes cells to accumulate protoporphyrin IX (Proto) and heme. Exposure to light in vitro causes intracellular Proto to initiate formation of singlet oxygen molecules, leading to self-destruction. This photoactivated destruction by ALA in vitro is enhanced by addition of the tetrapyrrole modulator 1,10-phenanthroline (Oph), which increases cellular accumulation of Proto. Here we significantly extend this idea by evaluating the efficacy of ALA and Oph photodynamic therapy of solid tumors in vivo. Methylcholanthrene-induced fibrosarcoma (Meth-A) cells were used, which lead to the formation of solid tumors when implanted into syngeneic recipients. Initially, suspensions of Meth-A cells were treated in vitro with combinations of ALA and Oph. Meth-A cells in suspension accumulated 6-fold greater amounts of Proto (P < 0.05) after 3-h incubation with ALA and Oph than when incubated with ALA alone, and were also more susceptible to subsequent photoactivated cell lysis in vitro. Similarly, solid Meth-A tumors grown in syngeneic BALB/c mice accumulated significant (P < 0.05) amounts of Proto 3 h after in vivo treatment with ALA, and Oph synergized with ALA to significantly (P < 0.05) enhance the induction of Proto in these tumors. ALA and Oph-based phototreatment of mice bearing Meth-A solid tumors resulted in necrosis of tumors, as determined by a significant reduction in both size and histopathology, with little damage to surrounding normal tissue. These data directly demonstrate the experimental usefulness of Proto modulators for ALA-based photodynamic therapy in the treatment of solid tumors in vivo and provide a rationale for their potential application in a multitude of tumor types.