The expression levels of mRNA for multidrug resistance 1 (MDR1) gene, multidrug resistance protein 1 (MRP1), lung resistance-related protein (LRP) and breast cancer resistance protein (BCRP), which confer multidrug resistance in vitro, were examined in 43 untreated breast carcinoma patients, of whom 38 subsequently received doxorubicin-based chemotherapy after surgery, in order to elucidate the roles of these genes in drug resistance in vivo. The mRNA levels were determined using a semi-quantitative reverse-transcription polymerase chain reaction method in breast carcinoma tissues including at least 80% carcinoma cells. The expression level of BCRP gene was low and did not vary markedly in comparison with that of MDR1, MRP1 or LRP gene. The expressions of MDR1 and MRP1 genes were correlated with each other, but the expression of BCRP or LRP gene did not correlate with that of other genes. These four gene expressions were independent of age, TNM categories and the status of progesterone or estrogen receptor. The expression levels of these four genes were not related to the relapse or prognosis of the 38 patients treated with doxorubicin-based chemotherapy. P-glycoprotein (P-gp) / MDR1, MRP1 and LRP may play more important roles than BCRP in chemotherapy of human breast carcinoma.