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Low-frequency ultrasound (LFUS) hasbeen usedwith good results in the treatmentof patients with peritonitis [i, 5]. The exposure dose used in such cases (6-8 sec/cm of peritoneum) is based on the absence of gross changes in the serous membrane [i, 2]. However, the present writers have shown clinically that with a duration of exposure to ultrasound as specified above, individual polymorphonuclear neutrophils (PNN) in the substance of the peritoneum and in the exudate are destroyed [4] and desquamation of mesotheliocytes takes place. At the same time it has been shown that LFUS can clear the peritoneal microvessels from thrombi and stasis and can activate emigration of PNN. Essentially no other information is available on the mechanism of the therapeutic action of LFUS in vivo. By analogy with high-frequency ultrasound it can be tentatively suggested that LFUS increases the powers of absorption of the peritoneum.LFUS, acting in vivo, can perhaps kill microorganisms present in the exudate and peritoneum, for such an effect has been found during long-term irradiation of a microbial suspension with ultrasound in vitro [2].For the reasons given above, in the present investigation mechanisms of action of LFUS were analyzed in the intact organism and during the treatment of peritonitis, and experimental evidencewas obtained on which to base an optimal duration of irradiation of the peritoneum and peritoneal exudate with ultrasound. EXPERIMENTAL METHODExperiments were carried out on 50 noninbred male albino rats weighing 260-300 g and on 190 guinea pigs of both sexes weighing 250-420 g. Ether anesthesia was used during any painful procedures.The peritoneum and exudate were irradiated with ultrasound at a frequency of 26.5 kHz and with an amplitude of 30 ~. Altogether five series of experiments were carried out.In the first two the duration of exposure of LFUS was studied. For this purpose the action of LFUS was studied on the peritoneum of intact animals (20 guinea pigs and 30 rats --series I) with exposures of between 20 and 120 sec. The phagocytic power of PNN and their chemotaxis also were studied in a modified Boyden's chamber [7] after different exposures of peritoneal exudate in vivo to ultrasound (series II). Exudate containing PNN was obtained by intraperitoneal injection of a 0.5% solution of amylodextrin 4 h before irradiation with ultrasound [3].In the remaining three series of experiments, the mechanism of action of LFUS in peritonitis were analyzed.In the experiments of series III, to determine the direct bactericidal action of LFUS a suspension of Staphylococcus aureus (strain Zhaev) or a 3% fecal suspension was injected into the peritoneal cavity and seedings of standard volumes of suspension (i. i0 -s ml) were taken before and after the action of LFUS.In the experiments of series IV the effect of LFUS on the ability of the peritoneum to absorb antibacterial preparations was determined.For this purpose, after exposure of guinea pigs to LFUS, tritium-labeled isoniazid (specific activity 168 mCi/mmole) was i...

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