The aim of the research: The aim of the research is to examine the potential cytotoxic effects of polysulfides, a chemical substance isolated from the Allium spp. family of plants on human carcinoma cells of urinary bladder (UM-UC-3) and glioblastoma (LN229). The assumption is that after the exposure of carcinoma cells to polysulfides their number will be reduced in comparison to the control group. Materials and methods: MTT assay was used to establish cytotoxicity levels on urinary bladder and glioblastoma cancer cell lines. The comparison between the absorbance of created formazan in cells treated with polysulfides and those untreated (the control group) established the survival rates of carcinoma cells. Cytotoxicity levels were established after the periods of 4, 24, 48 and 72 hours. Results: The results are graphically presented in relation to incubation times and the percentages of metabolically active cells. In urinary bladder carcinoma cells all polysulfides show better effects in higher concentrations of 50 μg/mL and 100 μg/mL, with different incubation times. The most prominent effect on the reduction of metabolically active cells is achieved by diallyl disulfide and dipropyl trisulfide at the concentration of 100 μg/mL after 72-hour incubation time. In glioblastoma cells most polysulfides have the strongest effect in higher concentrations of 50 μg/mL and 100 μg/mL, with different incubation times. The most significant effect on the reduction of metabolically active cells is achieved by diallyl disulfide and dipropyl trisulfide at the concentration of 100 μg/mL after 72-hour incubation time. Conclusions: In vitro exposure of urinary bladder carcinoma cells (UM-UC-3) and glioblastoma (LN229) to polysulfides leads to reduced cell survival rate. Cytotoxic effects depend on incubation time and concentration. Polysulfides effect is not always proportional to the increase in concentration and incubation time and in some cases cell recovery occurs. The cytotoxic effect of polysulfides is confirmed, which was the hypothesis of the research, and the next step is the confirmation of these effects by in vivo studies on the models of animal urinary bladder and glioblastoma cancers.