Bevirimat (PA103001-04) represents a new class of antivirals that blocks HIV replication by disrupting virus maturation; specifically, by inhibiting a late step in the Gag processing cascade. Short term (7-10 days) functional monotherapy studies (conducted in patients with detectable viral loads on a failing regimen)help in determining the potency of the drug, and enable dose finding. This is a two part (A and B)randomized, placebo-controlled, double-blind, multiple-dose, dose-escalation study in HIV treatment-experienced patients on a failing regimen (harboring resistance mutations to at least one member of the NRTI, NNRTI or PI classes. The antiretroviral activity, safety, and pharmacokinetics of up to 5 different dose levels of bevirimat will be compared to placebo when added to a failing approved antiretroviral regimen. The study is conducted in two parts: A and B. In Part A following 14 days of daily dosing patients commenced a new optimized ART regimen in addition to their randomized treatment. In Part Part B dosing with the randomized treatment ends after the initial 14 days of daily dosing.