Samples of serum inoculated with Escherichia coli and serum that became contaminated with bacteria after exposure to a laboratory atmosphere demonstrated elevated DNA polymerase activity. The levels of activity were well within the range of values found in hepatitis B e antigen (HBeAg)-positive samples. The bacterial polymerase activity was markedly reduced by a single passage of serum samples through a 0.22-micron Millipore filter prior to analysis. Repeated filtration did not result in a substantial further decrease in polymerase activity. In sera that were heavily contamined with E. coli, however, filtration was not successful in reducing bacteria-associated polymerase activity to a base-line uncontaminated level. In such instances double antibody immunoprecipitation proved effective in elimination of bacterial activity. When bacterial contamination of serum samples is a possibility, specimens should be subjected to either Millipore filtration or immunoprecipitation prior to analysis, particularly when correlation of DNA polymerase activity with HBeAg and its corresponding antibody is attempted.