Antibiotics commonly used in the treatment of pigs -amoxicillin trihydrate, oxytetracycline hydrochloride and thiamphenicol were added at different concentrations to aliquots of pig waste slurry plus anaerobic sludge in serum bottles. The biogas production and methane concentration in the headspace were monitored to determine the effect of the antibiotics on the anaerobic process. With thiamphenicol significant differences in methane production were found for concentrations of 80 and 160 mg 1(-1) slurry. Compared to the control, only minor differences in methane production were noted in the bottles to which amoxicillin (60 and 120 mg 1(-1)) had been added. Methane production was about the same for the bottles with different oxytetracycline concentrations (125 and 250 mg 1(-1)) and for the control.