The small molecular weight protein (p7) was overexpressed in the human ovarian carcinoma cell lines, SKVLB600 (selected for vinblastine resistance from SKOV3 cells). OVCAR 4/ADR100 (selected for doxorubicin resistance from NIH:OVCAR4) and (OVCAR4/VBL200 (selected for vinblastine resistance from NIH:OVCAR4). Trace amounts of the protein were also found in the parent cell lines, SKOV3 and NIH: OVCAR4. An anti-p7 monoclonal antibody (1D7) specifically inhibited the proliferation of the drug resistant cancer cells. In order to assess the function of p7, we established a new cell line, SKVLB600R, which was maintained in drug-free medium for 16 months. This cell line stopped expressing Pgp that is responsible for its resistance to cytotoxic drugs, but continued to overexpress p7. The proliferation of SKVLB600R was also inhibited by 1D7. Our results indicate that p7 may be specifically involved in the proliferation of multidrug-resistant cells.