Abstract Rod-shaped, membrane-bounded crystalline inclusions present in rabbit blastocysts were examined using electron microscopy and gel electrophoresis. Ultrastructural analysis reveals that the inclusions contain tubular elements which have the characteristics of microtubules including a wall composed of 13 subunits, or protofilaments. Unusual features of individual microtubules seen in negatively stained preparations are described. SDS polyacrylamide gel electrophoresis of inclusion-enriched fractions from blastocyst homogenates reveals a prominent band having a mobility identical with purified tubulin. The microtubules within the inclusions are considerably more stable than are cytoplasmic microtubules with respect to cold treatment and incubation in colchicine-containing solutions. The possibility that these inclusions might serve as a reservoir of microtubular protein for utilization in blastocyst growth and ovo-implantation is discussed.