In this study, water acidified with either hydrocholoric or sulfuric acid to pH levels of 2.0 or 2.5 was given for 6 weeks to normal and immuno-suppressed male, random-bred mice. The pH of the drinking water remained stable for periods up to 7 days. The only significant changes produced in both immunosuppressed and normal mice with both anions were declines in weight gain and water consumption when the drinking water was acidified to a pH of 2.0. Some of the observed changes were greater in the immunosuppressed animals. These animals also showed more variation in their response to the two anions. When sulfuric-acidified water at pH 2.0 was consumed, the number of bacterial species isolated from the terminal ileum was reduced. These findings suggested that the acidification of drinking water was not innocuous and that it should be evaluated as an environmental variable whenever it is used.