We present experimental results on the nucleation of nanoscale metallic lithium colloids, of well-controlled diameter (2–5 nm), in MeV electron-irradiated LiF single crystals. Conduction electron spin resonance experiments show a clear-cut quantum effect in these colloids: the spin susceptibility follows a mixed Curie–Pauli law, characteristic for tiny metallic particles in which the mean level spacing is comparable to kT. The behavior of the spin relaxation times ( T 1 and T 2) is presented and a discussion of quantum size effect in small metallic particles is proposed.