A case-control photodynamic therapy (PDT) was studied on adenocarcinoma (AC755) in BDF1 mice. Haematoporphyrin derivative (HPD; Porphyrin Products, U.S.A.) and a Bulgarian HPD were used as photosensitizers at doses of 10 mg kg-1. An argon dye laser system with lambda em=630 nm (400 mW cm-2) was used for PDT with a total light dose of 400 J cm-2. The therapeutic effect was assessed by the changes in tumour dimensions, the size of photonecrosis and the mean survival time of the animals. Histologic and ultrastructural studies were carried out. No significant difference was recorded between the antitumour effects of the two photosensitizers. Best results were obtained in small tumours (less than 10 mm) with incision of covering skin. Results are discussed in an attempt to advocate an optimal regimen for PDT in experimental tumours.