Even though the addition of silver sulfadiazine (AgSu) to purified deoxyribonucleic acid (DNA) results in the formation of AgSu-DNA complexes, no such complexes were detected in bacteria treated with AgSu. AgSu blocked macromolecular syntheses in treated bacteria, DNA synthesis being slightly more sensitive to this inhibitory action. The ribosomes, ribonucleic acid, and DNA isolated from treated cells were normal qualitatively. Bacteria deficient in DNA polymerase were not more sensitive than their parent strain to the lethal action of AgSu. Radioactive AgSu was localized mainly in the cytoplasmic membrane fraction of treated cells.