Commercial hematoporphyrin (Hp) and the tumor-localizing and photosensitizing agent hematoporphyrin derivative (Hpd) were analysed by means of high pressure lipid chromatography (HPLC). Furthermore, their efficiencies in sensitizing the photoinactivation of human cells in vitro were compared. The comparison showed that the least polar components of Hpd played the major role in this sensitization. In Hpd solutions used for injection in photochemotherapeutic treatment of cancer, these active components seem to be present as aggregates.