BACKGROUND Ultrasound/microbubble (USMB)-mediated sonoporation is a new strategy with minimal procedural invasiveness for targeted and site-specific drug delivery to tumors. The purpose of this study was to explore the effect of different breast cancer cell lines on sonoporation efficiency, and then to identify an optimal combination of USMB parameters to maximize the sonoporation efficiency for each tumor cell line. MATERIAL AND METHODS Three drug-sensitive breast cell lines - MCF-7, MDA-MB-231, and MDA-MB-468 - and 1 multidrug resistance (MDR) cell line - MCF-7/ADR - were chosen. An orthogonal array experimental design approach based on 3 levels of 3 parameters (A: microbubble concentration, 10%, 20%, and 30%, B: sound intensity, 0.5, 1.0, and 1.5 W/cm², C: irradiation time, 30, 60, and 90 s) was employed to optimize the sonoporation efficiency. RESULTS The optimal USMB parameter combinations for different cell lines were diverse. Under optimal parameter combinations, the maximum sonoporation efficiency differences between different breast tumor cell lines were statistically significant (MDA-MB-231: 46.70±5.79%, MDA-MB-468: 53.44±5.69%, MCF-7: 59.88±5.53%, MCF-7/ADR: 65.39±4.01%, P<0.05), so were between drug-sensitive cell line and MDR cell line (MCF-7: 59.88±5.53%, MCF-7/ADR: 65.39±4.01%, p=0.026). CONCLUSIONS Different breast tumor cell lines have their own optimal sonoporation. Drug-resistant MCF-7/ADR cells had higher sonoporation efficiency than drug-sensitive MCF-7 cells. The molecular subtype of tumors should be considered when sonoporation is applied, and optimal parameter combination may have the potential to improve drug-delivery efficiency by increasing the sonoporation efficiency.