The most important step in preventing the spread of cancer is to kill the malignant cells. Photodynamic therapy, a promising, new approach for destroying malignant cells, takes advantage of light, oxygen, and a drug (photosensitizer) that preferentially localizes in rapidly growing cells (1-3). A photosensitizer is any molecule that uses radiant energy or light to elicit a specific response. The most well-researched photosensitizers of photodynamic therapy are hematoporphyrin derivative (Hpd) and its active component, Photofrin II (porfimer sodium). It has been known since the 1960s that Hpd, which is formed by acid catalyzed acetylation of hematoporphyrin (Figure 1) (1, 2, 4) and subsequent alkaline treatment, preferentially localizes in the tumors of mice and rats and can be detected by its fluorescence. At the same time, scientists found that Hpd has photodynamic activity-it can augment or induce a toxic reaction when exposed to light (4).