INTRODUCTION: The purpose of the study was to determine the association of the microbiome with the emergence of bladder tumors caused by the consumption of BBN (N-butyl-N- (4-hydroxybutyl) nitrosamine). MATERIALS AND METHODS: The study was conducted on wild-type C57BL / 6J mice divided into 5 groups: an acute two-week BBN treatment model (“B” - mice consuming 0,05% BBN solution in drinking water for two weeks ), an acute treatment model of 2 + 2 weeks BBN / antibiotics (AB - two weeks pre-treatment with broad spectrum antibiotics followed by two weeks treatment with BBN solution (0,05%) and antibiotics in drinking water), mice on two weeks antibiotic treatment (A2 - two weeks mice treatment with broad-spectrum antibiotics exclusively), mice on a four-week antibiotic treatment (A4 - mice on a four-week treatment with broad-spectrum antibiotics only), and those who drink only water (which is divided into two experimental groups: positive K + and negative control groups K-). All measurements were performed on faeces samples by HPLC (High Performance Liquid Chromatography). RESULTS: The use of antibiotics led to a reduction in the number of bacteria involved in the conversion of BBN to BCPN ( N-butyl-N-(3-carboxypropyl) Nitrosamin). The concentration of BCPN present in the K + group (with the highest value of 1445,92 μg / ml) was significantly higher than in group A (where the highest value from the A2 group was 74,34 μg / ml). The measured BCPN concentration in the negative control is extremely low (4,58 μg / ml), similar to that measured in the A4 group. Faeces samples collected in the upper intestine (x̅ = 1266,89 μg / ml) had a higher concentration of BCPN than those collected in the lower intestine (x̅ = 371,26 μg / ml). CONCLUSION: In our study, we found that gut bacteria participate in the metabolism of BBN, more specifically, they have the ability to convert BBN to BCPN. We also concluded that the use of broad-spectrum antibiotics affects the significant reduction of the gut microbiome, respectively the bacterial gut culture involved in BBN metabolism, and that there is a difference in the extent of gut microbiome reduction depending on the duration of antimicrobial therapy. It is also important to point out that different parts of the gut differ in the type of bacterial cultures that inhabit them, more specifically the upper gut is more numerous with bacteria that have the ability to convert BBN to BCPN.