HT29 human colorectal cancer cells, from an immortal cell line, were grown to confluence on four 11 mm coverslips in minimal essential culture medium using 60 mm culture dishes. One dish was kept as a control. Following washing with serum-free medium, 1 mg of fluorescein isothiocyanate (FITC) conjugated with bovine serum albumin (BSA) was added to the remaining three dishes. FITC-BSA was added to dish 2. Sterile gas-filled microspheres (average diameter 3 μm) were added to dishes 3 and 4, in an approximate ratio of 230 microspheres to each cell. Dish 4 was aseptically insonated using 2.5 MHz pulsed wave diagnostic spectral Doppler ultrasound for 5 minutes. All cultures were then on-grown for a further 24 hours in minimal essential culture medium. Subsequently they were washed and dried with methanol, mounted using DPX medium and microscopically viewed. Dishes one, two and three showed low levels of background fluorescence, while dish four showed strong cellular fluorescence. The presence of microspheres during insonation facilitates the non-lethal entry of a large molecule (FITC conjugated BSA) into HT29 colorectal cancer cells.