Effectiveness of chemotherapy depends on the extent to which drug molecules penetrate tissues and cells to reach their intended target and limited by toxic side effects exerted by the therapeutic agent. The application of ultrasound and microbubbles has been shown to increase cell permeability and enhance intracellular delivery of cell-impermeable molecules, a process known as sonoporation. This work investigated the potential of ultrasound and microbubbles to enhance the therapeutic effect of a chemotherapeutic agent using an in vitro cell suspension system. The objectives were to measure clonogenic cell viability following treatment with ultrasound and microbubbles in the presence and absence of a chemotherapeutic agent, and investigate the effect of cell line, treatment order and acoustic pressure. Cells in suspension - breast cancer (MDA-MB-231) and prostate cancer (PC3) - were treated with ultrasound and microbubbles (USMB) at settings of 500 kHz pulse centre frequency, acoustic pressure (240 and 580 kPa peak negative pressure), 32 μs pulse duration, 3 kHz pulse repetition frequency, 30 s insonation time and 0.5% v/v of microbubbles (DA04, Artenga Inc.) in the presence and absence of chemotherapeutic agent docetaxel (Taxotere®). Cells were treated with docetaxel for two hours. The order of USMB and chemotherapy (CM) were varied. Following treatment, cell viability was assessed using clonogenic assay. Ultrasound and microbubbles combined with docetaxel increased MDA-MB-231 cell death by ~10 folds compared to chemotherapy alone. Clonogenic viability of ~2% was achieved with the combined treatment (CM+USMB at 240 kPa; chemotherapeutic agent was added prior to USMB treatment) compared to -20% with chemotherapy alone (0.01 nmol/mL). Whereas with USMB+CM (chemotherapeutic agent was added five minutes following termination of ultrasound), cell viability of ~5% was achieved. Comparable results were achieved at higher acoustic pressures. PC3 cell viabili- - ty of ~55% was achieved at 0.5 nmol/mL compared to ~65% at 0.0001 nmol/mL docetaxel, indicating that MDA-MB-231 was more sensitive to docetaxel compared to PC3 cells. In PC3 cells, a viability of -40% was achieved with the combined treatment, independent of the treatment order, compared to chemotherapy (~55%) and USMB (-90% at Pneg=240 kPa) treatments. Ultrasound and microbubbles enhanced the therapeutic effect of docetaxel, a chemotherapeutic agent, with a more pronounced enhancement in the chemosensitive cell line (MDA-MB-231) compared to the chemoresistant cell line (PC3). Future work will investigate this effect in vivo.