P-glycoprotein expression was investigated in ten drug resistant patients suffering from multiple myeloma. The immunoperoxidase method on bone marrow plasma cells and immunoblotting were performed by using the monoclonal antibody C219. Five patients resistant to protocols including vincristine, melphalan and doxorubicin were found to express P-glycoprotein. The immunoperoxidase method appeared to be sensitive and suitable for P-glycoprotein detection in bone marrow samples.