The effect of pulsed low-intensity ultrasound (PLIUS) on human articular chondrocytes was evaluated in an in vitro 3-D agarose gel culture model. Chondrocytes isolated from young children's articular cartilage of ablated polydactylia were embedded in gel after expansion and exposed to PLIUS on the third day after embedding. Another group of cells was exposed to sham PLIUS as a control. Different intensities of PLIUS treatment-18 mW/cm(2), 48 mW/cm(2), 72 mW/cm(2) and 98 mW/cm(2) (1.0 MHz, with burst duration of 200 micros repeated at 1.0 kHz)-were administered for 20 min/d, and the medium was replaced twice a week. The cultures were evaluated for aggrecan synthesis by enzyme-linked immunosorbent assay (ELISA), type II collagen production by Western blotting or ELISA and cell proliferation by total DNA measurement. The PLIUS was found to increase aggrecan synthesis in a time-dependent manner. The maximal response was observed at an intensity of 48 mW/cm(2). After 14 d of exposure at this intensity, the aggrecan synthesis was 214 +/- 26% of control, and type II collagen synthesis was 148.5 +/- 8.0% of control. However, PLIUS treatment revealed no significant influence on cell proliferation, confirming that the stimulation of aggrecan and type II collagen synthesis by PLIUS was not the result of an increase in chondrocyte cell proliferation. In addition, it was found that human chondrocytes harvested from older donors become less responsive to PLIUS. From this in vitro 3-D study of cultured human chondrocytes, our findings suggest that PLIUS may be applied to the tissue engineering of cartilage constructs.