Hematoporphyrin derivatives at an intracellular concentration in TA-3 mouse mammary carcinoma cells of 0.6 or 0.9 mM required input of 3.0-3.6 .times. 109 quanta/cell of red light (620 nm) to achieve a 90% kill. At an intracellular concentration of 1.2 mM, this light requirement drops to 1.5 .times. 109 quanta/cell. The energy for this photodynamic process is about 100 times higher than that required for ionizing radiation to achieve the same level of kill for these cells. The quantum yield for singlet O2 formation (the cytotoxic agent in most photodynamic processes) from hematoporphyrin derivatives is 0.75 .+-. 0.07 in ethanol but only 0.16 .+-. 0.07 in TA-3 cells, indicating possible intracellular complexing and quenching.