Photoradiation therapy of cancer in the presence of haematoporphyrin derivative is based on a retention of porphyrin in malignant tissue. After long term incubation of NHIK 3025 cells in the presence of 25 microgram ml-1 haematoporphyrin derivative, one fraction is easily removed from the cells by washing with a serum-rich medium. Another fraction remains bound to the cells for a prolonged time. The former does not contribute to the photosensitivity of the cells while the latter, the tightly-bound component, results in a photosensitivity proportional to the cellular contents of porphyrin. Transformed cells are shown to be slightly more sensitive and to retain 25-50% more haematoporphyrin derivative than non-transformed cells. Cytological effects of light absorbed by the tightly-bound component have been studied. The growth of treated cells is similar to that of control cells after a dose-dependent post irradiation lag period. A relatively slow leakage of lactate dehydrogenase (LDH) out of the cells takes place after treatment. The treatment induces a significant increase in the frequency of sister chromatid exchanges (SCE). We conclude that photoactivation of the tightly-bound fraction of haematoporphyrin derivative induces less damage to the outer cell membrane and probably more intracellular damage than irradiation of cells after a short period in contact with the derivative.