Suspended cells of a human lymphoblastic cell line were exposed to pulsed ultrasound of 775 kHz. The pulse lengths were varied between 16 and 1000 microseconds. The mark/space ratio was always kept at 1:1. Two ultrasound intensity levels were used: 3.6 and 6.4 W/cm2 spatial peak and temporal peak. After an exposure time of 5 min, cell membrane damage was measured cytometrically by a dye exclusion test. No membrane damage was observable at 16 microseconds, whereas, at pulse lengths of 1000 microseconds, about one-third of the cells were damaged.