Antigens associated with the conoid and apical pellicle in Apicomplexan parasites have previously been defined by a panel of monoclonal antibodies that recognize distinct but overlapping protein subsets (Morrissette et al. 1994). To probe the association of these antigens with the apical membrane skeleton of Toxoplasma gondii, we examined their distribution in detergent-extracted tachyzoites and in intracellular parasites treated with drugs that disrupt microtubules. Although intact parasites are labeled only at their extreme apex, immunogold localization in detergent-extracted tachyzoites indicates that the antigens under study are associated with the subpellicular microtubules along their full length. Brief treatment with oryzalin, which results in dissociation of subpellicular microtubules from the pellicle, permits identification of the apical antigens farther along the length of the parasite. Longer dinitroaniline treatment completely eliminates the subpellicular microtubules, after which the apical antigens are found as insoluble patches associated with the parasite pellicle. Taken together, these observations argue that the apical antigens under study are present along the entire microtubule in intact parasites but are inaccessible to antibody binding. In turn, this suggests that these antigens may play a role in the close association of microtubules with the parasite pellicle.