The paper is a discussion of research at Oregon State College in methods of determining permissible concentrations of pulp mill wastes in receiving waters by means of bioassays and application factors. Standard and convenient toxicity bioassay methods are being developed, and application factors for making use of the results of these bioassays are being determined experimentally in part by means of 'artificial streams' approximating natural conditions. Pulp mill wastes, whose acute toxicity has already been evaluated by means of bioassays, are introduced continuously in varying amounts into these streams for periods as long as one month. A similar approach is being developed for the control of the discharge of pulp mill wastes so that resulting 'slime' production can be maintained at biologically harmless levels.