Many established facts and valid conclusions pertaining to biological criteria by which the intensity of pollution can be judged already have been fully and clearly presented and need not be restated. However, some strikingly contradictory views are frequently encountered. Therefore, the present general discussion, based on a critical survey of pertinent literature and personal experience in the field of pollution control, may be helpful in the planning of biological studies. Bacteriological investigations and methods are not considered. (Author Modified Abstract)