Twelve cases of pure ovarian teratoma were studied by immunohistochemical methods to determine the incidence and localization of alpha-fetoprotein (AFP) production. There were six mature teratomas (grade 0) and six immature ones (grades 1-3). Three tumors (grades 1, 2, and 3 and stages III, I, and III respectively) had isolated foci of AFP immunoreactivity in yolk-sac-like vesicles, intestinal-type epithelium, or tissue resembling liver, whereas the other nine tumors had no AFP-positive components. The former three patients had preoperative serum AFP determinations; the values were elevated in two of them, and in both instances the oncofetoantigen became undetectable one month after oophorectomy. Both patients are alive and well 15 and 25 months later; the third patient with a normal serum AFP before surgery and a positively stained tumor died of recurrent disease. The authors' findings indicate that the production of AFP in pure ovarian teratomas recapitulates its pattern of synthesis during ontogeny. Furthermore, it is suggested that a preoperative elevation of serum AFP in a pure ovarian teratoma does not seem to correlate with stage or grade, beyond minimizing the possibility that the tumor is grade 0, or mature.