The International Geomagnetic Reference Field (IGRF) for 1900–2000 is testedin this paper. A peculiar behaviour is found in the time variations of higher-degree coefficients of the IGRF models. During the 1945–1955 period the spherical harmonic coefficients g n m and h n m with n > 7 show unusual jumps, while their variations before and after this period are quite smooth. These irregular variations have little effect on the main features of the surface magnetic pattern. However, when we extrapolate the field pattern downward through the insulating mantle to the core-mantle boundary (CMB), the contributions of the higher-degree coefficients become more important and are likely to affect the shape of the geomagnetic energy spectrum, distribution of magnetic flux, and magnetic determination of the conducting core radius. It seems necessary to re-examine and revise these coefficients in the IGRF models.