RP215 monoclonal antibody (Mab) was shown to recognize a specific carbohydrate-associated epitope found in cancer cell-expressed glycoproteins, known as CA215. The membrane-bound and soluble forms of CA215 were detected in almost all of the cancer cells in humans, but rarely found in normal tissues. Through MALDI-TOF MS analysis, it has been reported previously that as much as 40% of the detected tryptic peptides of CA215 showed high degrees of sequence homology to those found in immunoglobulin heavy chains. The cancer cell-derived immunoglobulins were further purified from CA215 by affinity column-linked with goat anti-human IgG for molecular characterizations. Semi-quantitative RT-PCR was used to determine the mRNA levels of various immunoglobulin genes expressed by cancer cells of single or multi-cell origins and compared with those found in normal human serum. The stability of CA215 was investigated under different experimental conditions. It was observed that the RP215-specific epitope in CA215 is stable at neutral pH, in human serum or in mice (half life of 5-18 days), but unstable at extreme pH's (pH ≤ 2.0; pH ≥ 12.0) or high temperatures. Enzyme immunoassays were performed with several secondary antibody probes related to human IgG. It was demonstrated that cancer cell-expressed immunoglobulins with RP215-specific epitope have much lower immunoactivity than that of normal human IgG (≤ 5%), despite the fact that both showed almost identical amino acid sequence in the respective Fc region reported previously. This could be the result of aberrant glycosylation of CA215 in cancer cells. Aberrant glycosylation of glycoproteins may have important biological implications on the proliferation of cancer cells in vitro or in vivo.