SUMMARYThe lysosomal enzymes, acid‐phosphatase and β‐glucuronidase, were released from rat liver lysosome when exposed to 400 nm irradiation in the presence of haematoporphyrin, and the release was prevented by adding vitamin E., diazabicyclo‐octane, bovine serum albumin, superoxide dismutase ord‐mannitol to the reaction mixture. Monochromatic irradiation with wavelengths from 380 to 410 nm caused no significant differences in the release of lysosomal enzymes, but 420 nm irradiation caused three fifths of that of 400 nm irradiation. The malondialdehyde level in rat liver homogenate increased after 400 nm irradiation in the presence of haematoporphyrin. Reduction of nitroblue‐tetrazolium was not observed when haematoporphyrin was excited by 400 nm; it was considered that superoxide anion radical (O–2) was not primarily generated.The following mechanism was assumed: that porphyrin which had been excited by 400 nm, converted ground‐state molecular oxygen (3O2) to excited singlet oxygen (1O2), which formed lipid peroxides in lysosomal membrane resulting in destruction of the membrane; skin changes would occur from these released lys