Resistance to several cytotoxic agents (MultiDrug Resistance MDR), including anthracyclines, vinca alkaloids and epipodophylline derivatives can occur in human osteosarcoma (OS) cells, detected by the overexpression of a 170 kD glycoprotein (p170), as a result of increased expression of the MDR gene (mdr1). The p170 glycoprotein in normal cells is a membrane transport system protein and its quantitative increase results in increased drug efflux and decreased intracellular drug concentration. Normal renal epithelial cells express p170 as a function of their secretory duties therefore this human tissue was used as a positive tissue control in our immunocytochemical study. This partially retrospective immunocytochemical study was carried out on routine, 10% neutral formalin fixed, decalcified, paraffin embedded, tissue sections of 43 OSs, treated between 1981 and 1993 at the Orthopaedic Hospital of Los Angeles. The immunoperoxidase antigen detection protocol, submitted by Hsu et al (1981) was employed. The search for p170 was carried out with three newly developed monoclonal antibodies (MoABs)(JSB-I, C-219 and C-494, from Signet Laboratories, Dedham, MA 02026). The initial expression of MDR was not detectable in seven OSs. 36/43 OSs expressed p170 on/in their cells. Heterogenous cellular microenvironment and various grades of differentiation features were also determined in the examined OSs. In 17/43 OS cases presence of intensive staining (probably overexpression) of p170 protein was registered. The 43 OSs exhibited different staining patterns with each MoAB. MoAB JSB-I reacted with a transmembranic antigen epitope. The long incubation time with C-219 resulted in heterogeneous cytoplasmic staining. MoAB C-494 also produced an intensive staining mainly localized on the cell membrane of the OS cells. These statistically significant immunocytochemical results suggest a direct correlation between the quantitative presence of p170 glycoprotein in human OS cells and the efficacy of the employed chemotherapy. Future observations employing the in situ hybridization technique will allow the quantitative measurement of the primary or secondary presence of MDR glycoprotein in human OS cells.