Abstract. In this note we consider the blowing-up X of P 3 along r general points of the anticanonical divisor of a smooth quadric in P 3 . Given a complete linear system L = |dH − m 1 E 1 − · · · − mrEr| on X, with H the pull-back of a plane in P 3 and E i the exceptional divisor corresponding to P i , we give necessary and sufficient conditions for the very ampleness (resp. base point freeness and non-speciality) of L. As a corollary we obtain a sufficient condition for the very ampleness of such a complete linear system on the blowing-up of P 3 along r general points.