cells. While the basis of this ephemeral invasiveness is not completely understood, we demonstrate here that it depends in part on the disruption of microneme secretion. Raising the temperature of freshly harvested parasites to 378C effectively triggered basal release of micronemal proteins into the medium. However, after a single such temperature cycle, both secretion of micronemes and cell invasion were abolished. Importantly, supernatants containing soluble microneme proteins were unable to restore cell invasion when added back to parasites that were refractory to further secretion. This result indicates that only parasites that are capable of active microneme secretion are competent for cell invasion. Impairment of microneme