P-170 glycoprotein is the phenotypic marker of multidrug resistance (MDR), and its detection may have relevance in identifying patients at risk of chemo-resistance. The expression of P-170 glycoprotein has been analyzed by the APAAP technique and monoclonal antibody C219 (which recognizes a cytoplasmic epitope of P-170) on bone marrow smears from 20 patients affected by responsive multiple myeloma. The study was performed longitudinally in the different phases of the disease, with specific regard to the remission phase. One of the patients evidenced a small number of P-170 positive plasma cells at diagnosis. Three of the patients showed scattered P-170 positive plasma cells during remission, which were also often positive for the nuclear proliferation-associated antigen recognized by monoclonal antibody Ki 67, as demonstrated by double immunostaining; all these subjects rapidly relapsed, expressing a MDR phenotype and resistant disease. Among the remaining patients, 5 are still in remission phase, 6 have relapsed without the MDR phenotype, achieving a second response to chemotherapy, 6 have had a resistant relapse with more than 80% of P-170 positive plasma cells in 3 cases. The presence of P-170 positive plasma cells during remission phase in multiple myeloma might identify a group of patients with high risk of early, resistant relapse.