The first part of this paper reports an experimental investigation of some parameters that control the occurrence of biologically effective cavitation‐type phenomena in the frequency region 0.25 4 MHz. The experimental criterion by which “cavitation” is recognized here is the simultaneous occurrence of degradation of DNA in aqueous solution, liberation of freeI2from KI solution, and generation of half‐order subharmonic. The intensity threshold for cavitation rises with frequency from 0.14 to 17.5 W cm−2(spatial peak) in the above frequency range. Pulse duration and duty factor are also found to be determining parameters, with cavitation activity reducing to zero when values of both parameters are sufficiently low. A strong positive correlation between ambient pressure and intensity threshold has also been determined. On the basis of observations of the levels of scattered half‐order subharmonic signals from various media, no evidence could be found for cavitation activity in sonicated mammalian tissues, although such activity could be detected in a wide range of liquids. The second part of the paper consists of a review, conducted in the light of the above results, of published experimental data relating to the existence and values of exposure thresholds for possible nonthermal, noncavitational mechanisms of action of ultrasound in living cells and tissues.