F. Gesztesy, K. A. Makarov, and E. Tsekanovskii An Addendum to Krein's Formula (35K, LaTeX) ABSTRACT. We provide additional results in connection with Krein's formula, which describes the resolvent difference of two self-adjoint extensions $A_1$ and $A_2$ of a densely defined closed symmetric linear operator $\dot A$ with deficiency indices $(n,n),$ $n\in \bbN \cup \{ \infty \}$.~In particular, we explicitly derive the linear fractional transformation relating the operator-valued Weyl-Titchmarsh $M$-functions $M_1(z)$ and $M_2(z)$ corresponding to $A_1$ and $A_2$.