Knowledge of the magnitude of the peak negative pressure, P(thr), at which ultrasound contrast agents fracture is relevant for using these microbubbles both as devices for contrast enhancement purposes, as well as carriers of drugs to be delivered locally. In the second part of this communication, the acoustic properties of three types of microbubbles stabilized by poly (vinyl alcohol) (PVA) shells are further investigated. In particular, the dependence of P(thr) on system parameters such as the number of cycles, frequency and exposure is examined. The effects of temperature, blood and, wherever data are available, of the dimension of the microbubbles on P(thr) are also considered. The large shell thickness notwithstanding, the results of this investigation show that at room temperature, PVA contrast agents fracture at negative peak pressure values within the recommended safety limit. Furthermore, P(thr) decreases with increasing temperature, radius of the microbubbles and number of cycles of the incident wave. Fatigue seems to be a physical mechanism playing a dominant role in the fracture process. The effect of blood on P(thr) varies according to condition under which the microbubbles have been synthesized, although stiffening of the shell is observed in most cases. In conclusion, these results suggest that PVA-shelled microbubbles may offer a potentially viable system to be employed for both imaging and therapeutic purposes.