Pulsed ultrasound has become a common therapy for delayed unions and nonunions after fractures. Experiments using ATDC5 cells, a mouse clonal chondrogenic cell line, have shown that 1 kHz dynamic acoustic radiation force stimulates proteoglycan synthesis similar to 1.5 MHz pulsed ultrasound. The results of two experiments performed 8 months apart showed that chondrocytes treated with 1 kHz squarewave had a 2 to 4-fold (p<0.01) increase in total area of nodules compared to control. When the same experiments were repeated 2 months later with the same frozen cell stock, the cells were unresponsive to mechanical stimulation, suggesting the cell line had transformed and was unable to differentiate into chondrocytes. This study demonstrates the need for primary chondrocytes to further examine the biological effects of 1 kHz vibration.