To examine the safety and tolerance of the administration of ganciclovir and foscarnet given together or alternately; to determine the interactive pharmacokinetics (blood level) profile of long-term combined and alternating therapy with these two drugs. Additional objectives are to examine the effect of these treatments in controlling time to cytomegalovirus (CMV) retinitis progression and to examine the antiviral activity of combined and alternating ganciclovir/foscarnet treatment and development of antiviral resistance. Sight-threatening CMV retinitis occurs in at least 6 percent of AIDS patients. By 1991 (US), there may be 6000 to 10000 patients with CMV retinitis. Many clinical reports suggest that both ganciclovir (DHPG) and foscarnet have an antiviral effect against CMV that is often associated with clinical stabilization. Effectiveness of ganciclovir and foscarnet is correlated with weekly maintenance and since toxicity is dose-limiting in up to 20 percent of patients receiving either drug for long periods, it may be beneficial in long-term maintenance treatment to combine or alternate these two drugs at a lower total weekly dose of each drug.