This is an initial descriptive report of observations of multiple forms of an organism found in patients with gestational trophoblastic disease and in patients with preeclampsia-eclampsia. The worm-like forms most frequently observed have an average length of 1.0 to 1.5 mm. Larva-like forms have an average length of 150 mu; primordial eggs and egg-like forms in developmental stages range from 7 to 43 mu in diameter; and sperm-like forms are 3.5 mu or slightly smaller in size. These forms have been observed in contact smears prepared from 3 ml samples of peripheral circulating blood from both groups of patients, from trophoblastic tumor tissue, from contact smears prepared from placentas of patients with preeclampsia-eclampsia, and from umbilical cord blood of infants delivered of patients with preeclampsia-eclampsia. The various forms of this organism share morphologic characteristics of several orders of helminths, i.e., hookworms, roundworms and tapeworms. The taxonomy of these forms has not yet been determined. Until the time of taxonomic classification, the various forms will be referred to as Hydatoxi lualba. We have experimental evidence that this organism has biologic activity in BALB/c mice and in beagle dogs.