The tissue distribution of Klebsiella pneumoniae 03 lipopolysaccharide (KO3 LPS) was studied in mice injected subcutaneously (s.c.) or intraperitoneally (i.p.) with '251-labeled K03 LPS. Marked retention of K03 LPS radioactivity could be found at the site of s.c. injection for several weeks. On the other hand, about 85% of the radioactivity rapidly disappeared from the peritoneal cavity within 6 h after i.p. injection. The long-term presence of K03 LPS at the injection site was also supported by experiments with 5'Cr-labeled K03 LPS and immunoblotting and immunofluorescence staining methods. The R-form LPS lacking the 0-specific polysaccharide chain of K03 LPS and the lipid A fraction of K03 LPS seemed to remain at the site in larger amounts and for longer times than K03 LPS. There were no marked differences in the retention pattern at the injection site among K03 LPS, Escherichia coli LPS, Salmonella typhosa LPS, and Salmonella enteritidis LPS. However, much less radioactivity accumulated in the livers and spleens of mice injected with either K03 LPS or S.on August 3, 2020 by guest