Objective This study compares the anti-tumor effects of single and combination use of clarithromycin and tamoxifen on estrogen receptor (ER) positive breast cancer cell lines,BT-483 and MCF-7 as well as triple negative cell line,MBA-MD-231,which acts as a negative control.The effect of solid breast tumor inhibition by clarithromycin is also studied.Method BT-483,MCF-7 and MBA-MD-231 were cultured in 6-well plates in a 37 ℃ humidified incubator without CO2 for 24 h prior to the addition of the test drugs.The test groups were clarithromycin ( Group 1 ),tamoxifen ( Group 2 ),clarithromycin and tamoxffen ( Group 3 ),and control ( Group 4 ).Group 3 was prepared in 1 to 1 ratio at a concentration of 1.5 mmol/L clarithromycin and 25 μmol/L tamoxifen.On the other hand,1 mm3 solid breast tumors were submerged into various groups as above for 24 h.On the harvest day,the proliferation of cancer cells and solid breast tumor samples were measured by WST-1 proliferation reagent while ATP bioluminescence assay was employed to measure the metabolic rate of the three cell lines.Results The proliferation of BT-483 and MCF-7 was suppressed most by combination use of clarithromycin and tamoxifen with statistical significance.The two drugs did not have an inhibitory effect on the hormonal negative cancer cells.For solid breast tumor samples,all the test groups showed reduced metabolic rate as compared with the control group ( P＜0.05 ).Conclusion Combination use of tamoxifen and clarithromycin are effective in suppressing cell proliferation and metabolism rate of breast cancer cells while single use of clarithromycin effectively inhibits the proliferation of solid breast tumor.rn