The paper deals with the operator $u \rightarrow gu$ defined in the Sobolev space $W^{r,p}(\Omega)$ and which takes values in $L^p(\Omega)$ when $\Omega$ is an unbounded open subset in $R^n$. The functions $g$ belong to Morrey type spaces which provide an intermediate space between $L ^\infty(\Omega)$ and $L^p_{loc}(\Omega)$ . The main result is an embedding result from which we can deduce a Fefferman type inequality. $L^p$ estimates and a compactness result are also stated.