Mammalian multicellular tumor spheroids (MTS), grown in liquid culture medium, 0.23 to 0.32 mm in diameter and consisting of 4,500 to 12,100 cells, were exposed to three ultrasonic machines: a clinical diagnostic device with an emitted intensity of 13 mW/cm2, a therapeutic machine with emitted intensities of 1 to 3.5 W/cm2, and a laboratory emitting 12 to 50 W/cm2. (All intensities are temporal and spatial average.) Four measures of ultrasonic effect on MTS were made: decrease in diameter after treatment, damage to subsequent growth outright kill, and number of cells detached from the MTS. There was no loss in MTS size from diagnostic or therapeutic exposures. None of the exposures from any of the three machines caused any damage to subsequent growth or survival. There was no significant detachment of cells from the MTS by any of the diagnostic exposures (at 13 mW/cm2, out of 40 minutes). With the laboratory machine, a significant number of cells were detached, about 0.02% of the MTS (approximately one cell per MTS) per W/cm2 in a ten-minute exposure. Cells detached by therapeutic exposure increased linearly with exposure. On the average, 1 W/cm2 detaches about 0.5 cells per minute, per MTS, or about 0.006% of the cells in the MTS.