Previous studies have shown that a large fraction of the host cell deoxyribonucleic acid (DNA) polymerase I (EC 2.7.7.7) becomes associated with the cell membrane shortly after infection with bacteriophages T4 and T7. The present investigation of the bound enzyme revealed that the polymerase activity can be eluted from the membrane with chelating agents, and that the material thus obtained shows many properties that distinguish it from purified DNA polymerase I. These include its chromatographic behavior, sedimentation rate, sensitivity to anti-DNA polymerase I antiserum, and activity with synthetic and natural DNA primers. Several of these physical and biological parameters were shown to revert slowly during storage to those exhibited by the purified enzyme. Efforts to determine whether the unusual properties of the membrane enzyme resulted from its association with DNA failed to support that possibility. These observations suggest that either the cause or the result of membrane binding of DNA polymerase I is a transient change in conformation or structure of the enzyme, with a resultant change in its enzymatic activity.