The gene expression of human histiocytic lymphoma cell line U937 at 6 h after 1 MHz ultrasound treatment in the presence of Ar or N2O gas was examined by DNA microarrays. Of the 9,182 genes analyzed, only the keratin gene was identified as down-regulated in the cells exposed to ultrasound in the presence of N2O where no internal cavitation was observed. In contrast, five up-regulated and two down-regulated genes were identified in the cells exposed to ultrasound in the presence of Ar where internal cavitation was apparently observed. Six changes of the gene expression were confirmed by the semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR). Gene expression of heme oxygenase was augmented by a factor of 6.6 in microarray and by 4.0 by RT-PCR. These results indicate that internal cavitation increased the expression of genes responsive to oxidative stress in sonicated cells but non-inertial cavitation had minimal effects on gene expression. (C) 2002 Elsevier Science.