In the outer membrane of P. mirabilis, a peptidoglycan-associated lipoprotein (PAL) of apparent molecular weight 18,000 is present as a major protein. A fatty acid-containing polypeptide (lipopolypeptide; LPP) was isolated by digestion of the purified PAL with trypsin in the presence of 0.05% SDS. It was composed of 31 amino acid residues, an unidentified compound[X], and ca. 3 fatty acid residues. A lipooligopeptide (LOP) was also isolated after further digestion of LPP with trypsin in the absence of SDS. LOP was composed of 4 amino acid residues (Asx, 2Ser, Lys), a compound[X], and ca. 3 fatty acid residues. The C-terminal amino acids of LPP and LOP were determined as arginine and lysine, respectively. On the other hand, the N-terminus of PAL, LPP, or LOP could not be identified by conventional N-terminal analysis of PAL, LPP, or LOP could not be identified by conventional N-terminal analysis, indicating that the N-terminus is probably masked. These results indicate that LPP and LOP are derived from the fatty-attached amino terminal region of PAL.