This study was undertaken to assess the effect of sound stress on immune response to thymus-dependent and -independent antigen in ICR or C57BL/6 mice and Sprague-Dawley rats. The humoral immune response was evaluated by active or passive hemagglutination(HA) response to sheep red blood cells(SRBC), a thymus-dependent antigen and polyvinylpyrroridone(PVP), a thymus-independent antigen. The cellular immune response was evaluated by delayed hypersensitivity(DTH) reaction to SRBC. After animals were exposed to 3 or 4th daily sound stressors(83dB) for varying length of time before and/or after immunization, the primary and/or secondary immune responses to SRBC and PVP were assayed. The primary and secondary antibody response of stressed mice to SRBC were significantly enhanced irrespective of exposure period to stressors relative to immunization. Titers of 2-mercaptoethanol resistant antibody in stressed mice after the secondary immunization was similar to or greater than those of control mice. The primary antibody response to PVP was similar to or slightly less than that of control mice when mice were exposed to the stressor for 2 days before or after immunization. While the primary antibody response to PVP was almost not detective when mice were exposed to the stressor for 4 days before or after immunization. Surprisingly, the secondary antibody response to PVP of the mice receiving the secondary sound stress was profoundly increased when immune-depressed mice received the secondary immunization at 46 days after the primary injection. DTH reactions of stressed mice was significantly suppressed compared with those of the unstressed mice when C57BL/6 mice were exposed to sound stress for 4 days before or after SRBC immunization whereas Arthus reactions and serum antibody responses of stressed mice to SRBC was similar to or somewhat greater than those of control. When rats were exposed to sound stress for 4 days before or after immunization, HA titers against SRBC of the stressed rats similar to those of controls, but antibody responses of the stressed rats were profoundly suppressed. The sound stress continued for 12 days did not alter the antibody response of rats of PVP and SRBC. The yield of thymocytes and thymus weight to the rats were decreased by the sound stressor. With these results, the authors may be able to conclude that the sound stress enhances the primary and secondary antibody responses to SRBC of mice and rats, but suppresses DTH reactions to SRBC, and that the sound stress profoundly depress the primary antibody responses but markedly increases the secondary antibody responses to PVP, and that the long length(12 days) of sound stress did not alter SRBC-and PVP-specific antibody responses.