We compared effects of ultrasound-induced vs. photodynamic cytotoxicit
y in cell culture. The photodynamic effects mediated by mesoporphyrin
caused a delayed toxic reaction, the presence of a ''shoulder'' on the
dose-response curve, indicating the capacity for limited repair of ph
otodamage. In contrast, ultrasound-induced loss of viability resulted
from rapid cell destruction and was proportional to the time of sonica
tion. Photodynamic damage to cells before exposure to ultrasound poten
tiated cell breakage but did not affect the clonogenicity of the survi
ving cell population. Photodamage after exposure to ultrasound decreas
ed the viability of cells which had survived ultrasonic treatment.