An experimental apparatus has been developed to determine thresholds for cavitation produced in a fluid by short tone bursts of ultrasound at 0.76, 0.99, and 2.30 MHz [A. Atchleyet al., Ultrasonics26, 280–285 (1988)]. A fluid jet was used to convect potential cavitation nuclei, such as 1‐μm polystyrene spheres, echo contrast spheres, and whole blood constituents, through the focal region of the insonifying transducer. Cavitation thresholds measured with this system in water and in a fluid with ten times the viscosity of water will be presented. Cavitation was detected by a passive acoustical technique that is sensitive to sound scattered from cavitation bubbles. Results from these experiments that permit the control of nuclei and host fluid properties will be compared to an approximate theory that predicts the onset of cavitation [C. K. Holland and R. E. Apfel, Trans. IEEEUFFC‐36(2) (1989)]. [Work supported by NIH, grant number 1RO1‐CA‐39374.]