Immunoconjugates are being explored as novel cancer therapies with the promise of target-specific drug delivery. The immunoconjugate, huN901-DM1, composed of the humanized monoclonal IgG1 antibody, huN901, and the maytansinoid drug, DM1, is being tested in clinical trials to treat small cell lung carcinoma (SCLC). huN901-DM1 contains an average of three to four DM1 drug molecules per huN901 antibody molecule. The drug molecules are linked to huN901 through random modification of huN901 at epsilon-amino groups of lysine residues, thus yielding a heterogeneous population of conjugate species. We studied the drug distribution profile of huN901-DM1 by electrospray time-of-flight mass spectrometry(ESI-TOFMS), which showed that one to six DM1 drug molecules were attached to an antibody molecule. Both light and heavy chains contained linked drugs. The conjugation sites in both chains were determined by peptide mapping using trypsin and Asp-N protease digestion. Trypsin digestion identified modified lysine residues, since these residues were no longer susceptible to enzymatic cleavage after conjugation with the drug. With respect to Asp-N digestion, modified peptides were identified by observing a mass increase corresponding to the modification. The two digestion methods provided consistent results, leading to the identification of 20 modified lysine residues in both light and heavy chains. Each lysine residue was only partially modified. No conjugation sites were found in complementarity determining regions (CDRs). Using structural models of human IgG1, it was found that modified lysine residues were on the surface in areas of structural flexibility and had large solvent accessibility.