The measured intermodulation product is compared with the calculation that follows the interpretation of the BCS-theory according to Dahm and Scalapino in postulating a specific current density and temperature dependence of the London penetration depth: λ(T,j)≈λ LT (T=0) 1−(T/T c ) 4 1−b Θ (T)(j/j c0 ) 2 −0.5 where j c0 is the current density at zero temperature and b Θ ( T) the nonlinearity factor. A good agreement between calculated and measured values b Θ ( T) is obtained if the theoretical d-wave temperature dependence is assumed for YBCO and s-wave behavior for niobium.