Doxorubicin shows a wide spectrum of activities in solid tumors, especially against breast carcinoma. The aim of this study was to examine if doxorubicin, when given at lower concentrations than applied in clinic, may induce changes in treated cells. With this purpose we developed human breast adenocarcinoma SK-BR-3 cell line resistant to doxorubicin. The sensitivity of these cells to doxorubicin and to some other cytostatics used in cancer treatment was determined by colorimetric MTT assay. Some parameters which may be of importance as prognostic factors in treatment of breast cancer were analyzed as well. The expression of genes involved in mitotic signal pathway (EGF, TGF alpha, EGF-R, erbB-2, erbB-3, c-myc and c-H-ras) was determined immunocytochemically. The concentrations of cathepsins were determined using quantitative immunoreactive assays (cathepsins B and L) or immunoradiometric assay (cathepsin D). The results revealed that even low doses of doxorubicin can induce numerous changes in treated cells: they become resistant to doxorubicin, and cross-resistant to several other cytostatics. The expression of the above mentioned genes involved in mitotic signal transduction, as well as cathepsins D and L, was similar in both parental and doxorubicin treated cells.