A linear relationship between the relative expansion of an off-resonance ultrasound contrast microbubble and low acoustic pressures is expected. In this study, high-speed optical recordings of individual phospholipid-coated microbubbles were used to investigate this relationship for microbubbles ranging from 2 to 11 microm and for acoustic pressures ranging from 20 to 250 kPa at a driving frequency of 1.7 MHz. For microbubbles larger than 5 microm, the relative expansion (DeltaD/D0) increased linearly with applied acoustic pressure, starting at the origin. The response of smaller microbubbles (<5 microm) also increased linearly with the applied acoustic pressure. However, linearity started at an acoustic pressure threshold value of 30 to 120 kPa for the different individual microbubbles. Below these pressure values, little or no oscillation was observed. The results may be explained by size-dependent mechanical properties of the phospholipid shells. An imaging technique such as power modulation imaging could profit from the presence of an acoustic pressure threshold in the microbubble response.