This study examined the effect of P85 (a pluronic block copolymer) and microbubble (MB) ultrasound contrast agents under ultrasound irradiation on gene transfection and expression. The pEGFP plasmids that can encode enhanced green fluorescent protein (pEGFP) served as a report gene and were mixed with different concentrations of MB/0.05% (w/v) P85. Then the plasmids were transfected into human hepatoma G2 (HepG2) cells. The HepG2 cells treated with MB/P85 or without treatment were exposed to ultrasound (US parameters: 1 MHz, 1.0 W/cm(2), 20 s, 20% duty cycle). Twenty-four hours later, the transfection efficiency was assessed by fluorescence microscopy and fluorescence activated cell sorting (FACS) analysis. The cell viability was evaluated by Trypan blue exclusion test. The results showed that the gene transfection efficiency in HepG2 cells under ultrasound irradiation was significantly higher than that without ultrasound irradiation. HepG2 cells in the MB or P85 group in the absence of ultrasound expressed less amount of green fluorescent protein. The expression efficiency reached (22.14 +/- 3.06)% and the survival rate was as high as (55.73 +/- 3.32)% in the 30% MB plus P85 group. It was concluded that MB and P85 in the presence of ultrasound can enhance gene transfection and expression.