Seventy-eight patients: 45 children, 33 adults and 27 normal healthy donors were enrolled in the study. Expression of P-glycoprotein (P-gp) was evaluated with three monoclonal antibodies (MAb's) directed to intra-(C219, JSB-1) and extra-cellular (MRK-16) epitopes of P-gp and immunocytochemical (IC) APAAP staining method. Twenty-seven healthy donors peripheral blood mononuclear cells (PBMC) were investigated by means of IC and FACScan analysis. Positive staining for P-gp was detected in 31% children's and 33% adults' leukemia samples. No reactivity of three MAb's was observed with peripheral blood mononuclear cells (PBMC) by means of IC. Flow cytometry analysis with C219 MAb revealed staining for P-gp present on sub-population of lymphocytes and monocytes. P-gp (+) as well as P-gp (-) cases were compared in respect to clinical outcome, FAB classification and blood group. Complete remission (CR) was achieved in 12/14 (85%) children's and 9/11 (81%) adults' P-gp (+) leukemia cases. Within the P-gp (-) leukemia cases CR was observed in 24/29 (82%) and 18/22 (81%), respectively. Partial remission, relapse, resistance and death were noticed in 14% children's and 18% adults' P-gp (+) samples. In P-gp (-) cases these parameters were observed in 17% and 18%, respectively. These results raise the question whether the expression of P-gp can be used as single prognostic marker to detect multidrug resistance (MDR phenomenon) in vivo?