Introduction: Reactivation of BKV infection caused by decrease of immunity associated with organ transplantation can lead to nephropathy and organ loss. Antiviral drugs such as ganciclovir or foscarnet did not display any activity against BK virus. A source of hope can be found in chemotherapeutics usually used in bacterial infections that inhibit the replication of viral genetic material in cells. One of the aforementioned chemotherapeutics is ciprofloxacin. The aim of the study is to evaluate the antiviral activity of ciprofloxacin against BK virus in vitro culture. Material and methods: Initially, the effects of ciprofloxacin on Vero cell viability were assessed with MTT. After determining the 3 largest non-cytotoxic concentrations of antibiotics, their effect on viral replication in cells was determined. 24-h cells culture in DMEM medium supplemented with 10% serum after 2h incubation with a 6.3 x 105 copies/ml infection virus were used to evaluate the effects of different concentrations of ciprofloxacin on the number of BKV copies. Quantification of BKV replication in the culture was performed by real-time PCR. Results: By means of the MTT test, the 3 largest non-cytotoxic ciprofloxacin concentrations were determined as follows: 31.25; 62.5; 125 μg/ml. Cells treated with ciprofloxacin at a concentration of 125 μg/ml display the greatest decrease in BKV replication that equaly 1.8 – 3 log/5.8 – 9.2 Ct, compared to the control of the virus without drugs. A decline in the number of copies of the virus was observed during the lower concentration of the antibiotic (62.5, 31.25 μg/ml) by 1.3 – 2 log/4 – 6.4 Ct and 0 – 0.8 log/0 – 2, 4 Ct, respectively. Conclusions: Ciprofloxacin exhibits antiviral activity against the BK virus by inhibiting its replication in the in vitro culture in Vero cell line.