Modification of macrophage phagosomes begins shortly after formation as Toxoplasma cells secrete membranous vesicles that form a reticulate network within the vacuole. The Toxoplasma-modified compartments then resist normal endocytic processing and digestion. We have used the pronounced Ca++-dependent stability of the intraphagosomal membrane (IPM) network to purify and characterize the structural proteins of this assembly. In addition to the structural matrix, Toxoplasma secretes a discrete set of soluble proteins, including a newly described 22-kD calcium-binding protein. The IPM network adheres to intact Toxoplasma cells 100,000 g. Purified IPM vesicles were specifically recognized by immune sera from mice with chronic Toxoplasma infection and consisted primarily of a 30-kD protein when analyzed by SDS PAGE. IPM network proteins share a major antigenic component located on the surface of extracellular Toxoplasma cells as shown by immunoperoxidase electron microscopy using a polyclonal antibody prepared against the IPM vesicles. Moreover, in Toxoplasma-infected macrophages, anti-IMP antibody confirmed that the extensive IPM array contains proteins also found on the Toxoplasma cell surface. Our results indicate the IMP network represents a unique structural modification of the phagosome comprised in part of Toxoplasma surface proteins.