The mutual-friction parameter B' has been measured in the temperature range 1.3 to 2.1°K with an error ranging from 20 to 100%. The method is to observe the removal of mode degeneracy by rotation of a resonator for second sound. The empirical temperature dependence of B' is in good agreement with the kinetic model of Hall and Vinen; but the agreement with all published calculations of the roton-vortex-line scattering cross section is poor. The results are at variance with the predictions of Lin's equations of motion for rotating helium II but are in accord with the equations of Hall and Vinen. Some measurements on the attenuation of second sound in a rotating resonator when the angular frequency of rotation is modulated show that there is very little interaction between the different types of vortex motion in the resonator.