Let (P) be the transition semigroup of a Lévy process (L) taking values in a Hilbert space H. Let ν and N~ respectively be the Lévy measure and compensated Poisson random measure of (L). It is shown that for any bounded and measurable function f,AqPtf(x)=1tE[f(Ltx)∫0t∫Hq(y)N~(ds,dy)]for all t>0,x∈H, where A is some nonlocal operator. As a corollary,∫H|Ptf(x+y)-Ptf(x)|2ν(dy)≤1tPtf2(x)for all t>0,x∈H. As ν can be infinite this formula establishes some smoothening effect of the semigroup (P). In the paper some applications of the formula will be presented as well.