OTS). The situation of a leak of sctiwe material into a flowing air- stream presents the problem of determining possible upstream diffusion of the activity. In a constant velocity one directional flow involving three dimensional diffusion from a point source, the upstream concentration is severely limited compared to that downstream. For all practical values of the flow velocity and diffusion coefficients, this upstream concentration is negligible. It is to be noted, however, that the velocity profile of a flow in a finite channel involves points of very low velocity, and at these points the diffusion effect might predominate, resulting in some upstream diffusion. Even in this case, unknown convection effects would be more important than diffusion effects. (auth)