  It is shown within the weak-coupling model that the macroscopic
superconducting anisotropy for materials with the gap varying on the Fermi
surface cannot be characterized by a single number, unlike the case of clean
materials with isotropic gaps. For clean uniaxial materials, the anisotropy
parameter $\gamma (T)$ defined as the ratio of London penetration depths,
$\lambda_c/\lambda_{ab}$, is evaluated for all $T$'s. Within the two-gap model
of MgB$_2$, $\gamma (T)$ is an increasing function of $T$.

Superconductivity