AMENDED: 04-04-91 On the first day of therapy, a loading dose is given. After the first day, patients take pyrimethamine or placebo 3 times a week. Patients also take leucovorin calcium orally three times weekly. Enrollment occurs over approximately 12 months. All patients are followed on study until a common study close-out date and final analysis of the study. It is anticipated that this common close-out will occur when the mean duration of time on study therapy will be 3 years (approximately in January, 1994).