Antibiotic resistance is one of the leading medical problems and it is believed that the environment contaminated with antibiotics contributes to this problem. The aim of this master thesis was to compare the mechanisms of resistance to macrolide antibiotics between bacterial isolates originating from the environment polluted with macrolides and clinically relevant bacterial pathogens (streptococci and staphylococci) from Croatian hospitals. 16S rRNA gene sequencing revealed higher diversity of environmental isolates originating from the discharge site of industrial wastewaters compared to the upstream (control) site, possibly due to input of wastewater bacteria and / or selection of bacteria from the sediment in response to the high selective pressure of macrolides. PCR detection of genes for macrolide resistance showed prevalence of three resistance mechanisms in isolates from the discharge site, which are mediated by ribosomal methyltransferases (mostly ermB and ermF genes), ribosomal protective proteins (mainly msrE gene) and macrolide phosphotransferases (mphE gene) compared to the isolates from the upstream site. Among the analyzed clinical isolates, the most common mechanism was efflux pumps, and similar to the isolates from the discharge site, other two dominant mechanisms included ribosomal methyltransferases and ribosomal protective proteins. Although most isolates had one mechanism of resistance to macrolides, environmental isolates from polluted site and clinical isolates possessed multiple resistance mechanisms possibly as the way of adaptation and survival of the bacteria in the environment with high concentrations of antibiotics. High similarity of nucleotide sequences of ermB gene between environmental isolates from polluted site and clinical isolates indicates the possibility of spreading of this gene outside the environment.