Ferromagnetic resonance has been observed in single crystals of the family La1−xPbxMnO3with 0.25 < x < 0.45 from 13 to 25 GHz and in the temperature range from 77 to 430 °K. The line width, ΔH, exhibits a sharp minimum accompanied by an isotropic shift in the resonance line at a critical temperature, Tc. Tc is dependent on the concentration of Mn4+ and has been related to the Curie temperature. The shift in the peak of the resonance line at Tc can be described by effects related to the formation of a spin-polarized conduction band. The line shape is asymmetric at high temperatures, ~Tc, and at low temperatures, T ~ 77 °K. This asymmetric behavior of the line is probably due to transport properties of the conduction-band electrons.