To determine whether there is a pharmacokinetic drug interaction between oral ganciclovir and oral zidovudine (AZT) and between oral ganciclovir and oral didanosine (ddI). To determine whether concurrent administration of probenecid affects the pharmacokinetics of oral ganciclovir. To obtain data on the short-term safety of oral ganciclovir administered concurrently with AZT, ddI, or probenecid in HIV-positive patients.