To evaluate the safety of single and multiple doses (28 daily doses) of 9-[2-(R)-[[bis[[(isopropoxycarbonyl)- oxy]methoxy]phosphinoyl]methoxy]propyl]adenine fumarate (PMPA) prodrug administered orally to HIV-infected patients. To determine the pharmacokinetics of single and multiple doses of PMPA prodrug when administered orally to HIV-infected patients. To evaluate the anti-HIV activity of PMPA prodrug, as demonstrated by increases in CD4 cell counts and decreases in HIV RNA, when administered orally as a single dose and daily for 4 weeks to HIV-infected patients with CD4 cell counts of 200 or more cells/mm3.