The results obtained previously for scalar and class P completely monotone relaxation moduli are extended to arbitrary anisotropy. It is shown for general anisotropic viscoelastic media that, if the relaxation modulus is a locally integrable completely monotone function, then the creep compliance is a Bernstein function and conversely. The elastic and equilibrium limits of the two material functions are related to each other. The relaxation modulus or its derivative can be singular at 0. A rigorous general formulation of the relaxation spectrum in an anisotropic viscoelastic medium is given. The effect of Newtonian viscosity on creep compliance is examined. Notation d space dimension; D = d(d + 1)/2; S the linear space of symmetric d × d matrices; G the set of symmetric operators on S; T d the set of linear transformations of a d-dimensional vector space V d ; A. Hanyga (B) 42 A. Hanyga, M. Seredyńska I unit operator on S; A transpose of A; v A w = v k A kl w l ; v, F w := v kl F klpq w pq ; R + =]0, ∞[, R + = [0, ∞[; C + := {z ∈ C | Im z > 0}; C − := C\ ] − ∞, 0]; L p (A; V) the space of measurable functions f : A → V with | f | p ∈ L 1 (A; R + ); M(A; V) the set of positive Radon measures on A with values in V; M + (A; T d ) the set of Radon measures on A with values in T d ; |H|(E) total variation of a Radon measure H ∈ M(A; V); σ, σ1D stress, stress tensor; e, e scalar strain, strain tensor; g, j scalar relaxation modulus and creep compliance; G, J tensorial relaxation modulus and creep compliance.