The aim of the research: The aim of the research was to examine the potential cytotoxic effect of colloidal silver on human bladder cancer cell lines, specifically T24 and TCCSUP lines. The hypothesis was that treatment with colloidal silver solutions will reduce the number of metabolically active cells compared to the control group. Materials and methods: Cytotoxicity was assessed using the MTT method, which measures the percentage of metabolically active cells after treatment with colloidal silver solutions of varying concentrations prepared in honey. The solutions were prepared at concentrations of 0.1%, 0.5%, 1%, 2.5%, 5%, 10%, 20%, 30%, 40%, and 50%, and the effects were observed after 4, 24, 48, and 72 hours. The effectiveness of colloidal silver was determined spectrophotometrically by measuring absorbance at 570 nm. Results: Results are presented graphically as the relation of the percentage of metabolically active cells and the incubation time and the concentration. Colloidal silver shows a significant cytotoxic effect on T24 bladder cancer cells at concentrations of 10%, 20%, 30%, 40%, and 50% during intervals of 24 to 72 hours, with the highest cytotoxic activity observed after 72 hours of incubation. Colloidal silver also exhibits cytotoxic effects on TCCSUP bladder cancer cells at concentrations of 20%, 30%, 40%, and 50% in intervals from 24 to 72 hours of incubation. Colloidal silver demonstrates a greater cytotoxic effect on the T24 cell line, in contrast to the TCCSUP cell line, which shows a smaller decrease in the number of metabolically active cells. Conclusion: In vitro exposure of T24 and TCCSUP bladder cancer cells to colloidal silver solutions of varying concentrations leads to a reduction in the number of metabolically active cells. Colloidal silver exhibits a cytotoxic effect that is dependent on concentration and incubation time. The cytotoxic effect of colloidal silver on human cancer cell lines, which is also the hypothesis of this study, has been confirmed. Further in vivo testing on animal models is necessary to validate the cytotoxic effect of silver.