An interpretation of the experimental data on the low-temperature conductivity and magnetic susceptibility of mercury selenide containing donor impurities of transition elements is developed on the basis of electron resonance scattering theory in the Friedel approach. Both existing data and results obtained in the present study, for solid solutions of chromium, cobalt, and gadolinium, are considered. The results of a fitting of the measured temperature dependence of the electron mobility in HgSe:Cr crystals and concentration dependence of the Curie constant in the impurity magnetic susceptibility of HgSe:Co crystals are analyzed to obtain quantitative confirmation of the idea that resonance donor levels of the impurities chromium and cobalt are present in the conduction band of the respective crystals. The resonance level widths are determined and are found to be an order of magnitude larger than those of iron. It is shown that the observed concentration maximum of the electron mobility in mercury selenide containing gadolinium impurities can be explained on the basis of resonance scattering theory, although the existing data are insufficient for justification of such an explanation of the origin of that maximum.