During its intracellular life cycle within the infected host cell, Toxoplasma gondii is able to undergo rapid asexual replication. Neither the mechanism by which the parasite initiates this process nor the requirements for maintaining it are understood. We produced a monoclonal antibody, 1B8, that identifies a parasite antigen of approximate M(r) 97 kDa as determined by SDS-PAGE. The epitope recognized by mAb 1B8 appears as a collection of vesicular structures scattered throughout the cell cytoplasm. When RH strain parasites are incubated with mAb 1B8 in the absence of serum complement, parasite growth is inhibited by > 90% as determined by radioisotope incorporation. Both attachment and invasion assays show that neither of these parasite-host cell interactions are inhibited by the mAb. However, a marked reduction in the number of intracellular rosettes was observed following mAb treatment of the parasites. Viable extracellular parasites are able to endocytose mAb 1B8. Once within the parasite cytosol the antibody recognizes the vesicular structures similar to those observed with fixed parasites. Immunofluorescence assays with Besnoitia jellisoni and Plasmodium falciparum show that the epitope recognized by mAb 1B8 is conserved among Coccidiae but not the kinetoplastid Leishmania.