Hematoporphyrin derivative (HpD) tumor cell killing efficiency has been evaluated as a function of power density, optical spectrum, HpD concentration, and HpD preparation using trypan blue exclusion assay. Cell survival curves for Mayo Clinic HpD and for Photofrin were equivalent at all concentrations and power densities for both violet and red light. Survival curves for red light demonstrated a two-fold increased cellular killing efficiency at higher densities (160 mW/cm2). However, for violet light an irreversible and marked decrease in cellular killing efficiency was found as the power density was increased. This phenomena of decreased cellular killing efficiency of violet light at high power densities is consistent with a process of photo-decomposition of HpD and is most pronounced at low concentrations (less than 25 micrograms/ml).