The relationships between the rate of post-irradiated photohaemolysis sensitized by chlorin e6 and parameters such as the light fluence (time of irradiation) and sensitizer concentration were studied. On the basis of the single-parametric approach proposed by Valenzeno and Pooler, it was found that the haemolytic rate varies with the square of both the light fluence and the sensitizer concentration. Thus it can be concluded that, in a single erythrocyte lesion, two chlorin e6 molecules participate, each absorbing one photon. The possibility of suppression of post-irradiation haemolysis was also studied using the lipophilic antioxidant, butylated hydroxytoluene (BHT), and scavengers of 1O2, O2.- and HO. radicals. It was found that BHT inhibits, to a considerable extent, the post-irradiation lysis of cells, by about a factor of 2.5 at a BHT concentration of 9 microM. The addition to the medium of NaN3 (a scavenger of 1O2), superoxide dismutase (a scavenger of O2.- radicals), ethanol and D-mannitol (scavengers of HO. radicals), when irradiation was interrupted, did not produce a marked influence on the kinetics of subsequent haemolysis. On the basis of the results obtained, the nature of erythrocyte targets, which are crucial for the photodynamic effect of chlorin e6, is discussed.