Electron spin resonance (ESR) line widths of Mn2+ in lime were measured in samples doped with small concentrations (up to 0.4 mol %) of Mn. The line width was found to change in a predictable way with Mn concentration, according to the theoretical equation DeltaH(pp) = 0.23 + 240f(1 - f)(42), where DeltaH(pp) is the line width-in milliteslas and f is the Mn concentration in mole percent. The experimental results show that the technique can be used to measure, rapidly and nondestructively, small concentrations of Mn in commercial CaO.