The effective resistivity eta a and viscosity mu a through which small-scale MHD turbulence affects large-scale magnetic structures in a low- beta plasma are derived. While mu a only depends on the turbulent magnetic field and is always positive, eta a becomes negative when the magnetic turbulence energy density exceeds the kinetic one. This generalizes a previous result in the theory of 2D MHD turbulence. The origin of a recently published result giving a positive definite resistivity is pointed out.