Endohedral fullerenes (M@C_2n) are carbon clusters that contain atoms trapped within the fullerene cage. This “atomic park” of the 21st century counts approximately 30 elements, mainly of 2nd group (Ca, Ba, Sr) and 3rd group (Sc, Y, Ln). Some of them, like La@C_2n, have unpaired electrons. For La@C_2n embedded in the polycarbonate film, we discovered an intense ¹H-ENDOR (electron-nuclear double resonance) signal. Since there are no protons in La@C_2n, this was a matrix ¹H-ENDOR that testifies permeability of the fullerene shell for the electron spin density. For liquid solutions of La@C_2n in hexamethylphosphoramide (HMPA), molecules of which contain NMR active phosphorus-31, the paramagnetic shift of ³¹P NMR of bulk HMPA molecules in the presence of La@C_2n was revealed. Thus, ENDOR and NMR testify that the area of localization of unpaired electrons is not restricted by the fullerene shell but a partial runoff of the electron spin density outside the fullerene cage (“spin-leakage”) takes place. Furthermore, M@C_2n form nanoparticles, the mean size of which ranges up to 100 nm in polar solvents. The “spin-leakage” leads to the partial loss of molecular paramagnetism of M@C_2n at forming the nanoparticles. The magic angle spinning (MAS) NMR of solid complexes of La@C₈₂ and Y@C₈₂ with HMPA revealed the enormous shifts for ¹³⁹La lines along with the vast range of the shifts for ³¹P of the bound HMPA. Being unchanged with temperature, such shifts suggest their Knight nature caused by the density of conduction electrons at the positions of the NMR observed nuclei.