The photodynamic action of hypericin (HYP) in vitro was evaluated using human leukemic HL-60 and lung carcinoma A549 cell lines. After illumination HYP (1 x 10 (-5) M) reduced the proliferation and/or survival of HL-60 and A549 cells vs. controls to almost to 0 % and 29 %, respectively. A lower concentration of HYP (1 x 10 (-6) M) decreased the proliferation and/or survival only in HL-60 cells. Non-cytotoxic concentrations of the carbonic anhydrase inhibitor acetazolamide (ACTZ) (1 x 10 (-3)-1 x 10 (-6) M) significantly potentiated these effects of HYP (1 x 10 (-6)M) in HL-60, but not in the A549 cell line. The highest concentration of ACTZ (1 x 10 (-3) M) also induced an increase of the subdiploid G 0 /G 1 population in HYP (1 x 10 (-6) M) treated HL-60 cells from 14 % to 24 %. The results indicate that the photogenerated pH drop may participate in the potentiation of the photodynamic action of HYP observed in leukemia cells.