Recently, it has been observed that drugs that induce the permeabilization of the lysosomal membrane (LMP) exhibit anticancer properties and are able to induce apoptose in tumor cells independently of p53 pathway. Phenothiazines are drugs extensively used for the treatment of schizophrenia and structurally have a thiazine nucleus with substitutions in the carbon 2 and nitrogen 10. Literature data have revealed that these drugs are able to induce apoptosis and to suppress cell proliferation in leukemic cells. In this work we investigate the involvement of lysosomal permeabilization in phenothiazines-induced apoptosis in human leukemic K562 cells. When K562 cells were incubated with the phenothiazines thioridazine (TR), trifluoperazine (TFP) and fluphenazine (FP) for 24 h, a decrease of cell viability assessed by MTT method was observed, with the IC₅₀ values ranging from 15 to 50 µM depending on the structure of the phenothiazine derivative. Confocal images of acridine orange-loaded cells incubated for 1 h with 50 µM phenothiazines demonstrated typical apoptotic morphological alterations and relocation of the probe indicating a disruption of lysosomes and release of their content to cytosol. Further experiments are necessary to determine the effects of LMP on caspases activation. Taken together, these results indicate that the phenothiazines are able to induce apoptotic cell death in human leukemic K562 cells with involvement of LMP presenting a pharmacological potential in cancer therapy.