Pharmaceutical production industries are one of the main sources of antibiotics, and they release considerable amounts of antibiotics to ecosystem. Antibiotics usually pres- ent as mixtures in treatment plants and have negative effect on biological processes.

In this study, batch acute tests were performed to assess the inhibitory impacts of selected antibi- otic combinations of sulfamethoxazole and tetracycline (ST), erythromycin and sulfamethoxazole (ES), and erythromycin and tetracycline (ET) on acetoclastic methanogenic activity. Each antibiotic was equally applied, making the total concen- trations in the mixtures 0 (control), 2, 20, 50, 100, 250, and 500 mg/L. Results showed decline characteristic on methane production with increasing antibiotic concentrations. EC50 values were calculated as 275 mg/L for ES, 219 mg/L for ST, and 130 mg/L for ET. Mixture inhibition of ST and ET combinations were accurately predicted using the concept of independent action, while ES combination resulted in almost the same inhibition with that of single antibiotic response. Inhibition on acetate utilization followed similar trend with methane production inhibition.