The peptidoglycan-associated lipoprotein (PAL) is present in the cell envelope in a form closely, but not covalently, associated with peptidoglycan in various Gram-negative bacteria. When the cell envelope or the isolated peptidoglycan-PAL complex from Proteus mirabilis, in which PAL maintains the interaction with peptidoglycan, was digested with trypsin, a polypeptide fragment with molecular weight 11,000 (11 K fragment) was obtained. However, when isolated PAL or the 11 K-fragment which had been dissociated from peptidoglycan was treated with trypsin, they were further digested. The 11 K-fragment maintained essentially the same tight interaction with peptidoglycan as intact PAL. These results indicate that the 11 K-fragment is probably derived from the peptidoglycan-associated region of the PAL molecule. The purified 11 K-fragment contained neither covalently-linked fatty acid nor glycerylcysteine, which are known to be present at the N-terminus of PAL. The N-terminal sequence of the 11 K-fragment was also determined.