In order to estimate interfering effects of mercury and lead on biochemical oxygen demand (BOD), BOD in 18 effluent samples were measured under three different concentrations of mercury and lead. The results obtained were as follows: 1. Biochemical oxygen demand(BOD) was decreased under the presence of mercury and lead, with parallel correlation of mercury concentration. 2. High correlations were noted between original BOD concentration and decreasing amount of BOD when concentrations of mercury or lead were increased. 3. When the lead concentration was high, the close correlation was observed between total organic carbon(TOC) and decreasing amount of BOD. 4. There was a negative correlation between TOC/BOD ratio and decreasing amount of BOD when the mercury concentrations were high.