This study explores new treatment options for ACTG 320 enrollees (and, if needed, a limited number of non-ACTG 320 volunteers) who have been receiving ZDV (or d4T) plus 3TC and IDV and are currently exhibiting a range of virologic responses. By dividing the study into the corresponding, nonsequential cohorts (Groups A, B, C, D), different approaches to evaluating virologic success, i.e., undetectable plasma HIV-1 RNA levels, and virologic failure, i.e., plasma HIV-1 RNA levels of 500 copies/ml or more [AS PER AMENDMENT 12/27/01: 200 copies/ml or more], are explored while maintaining long-term follow-up of ACTG 320 patients. [AS PER AMENDMENT 12/27/01: Groups B, C, and D completed follow-up on March 4, 1999. Therefore, only information pertinent to Group A is applicable. This study will examine the question of whether intensification of therapy can prolong the virologic benefit in individuals whose plasma HIV-1 RNA concentrations have been below the limits of assay detection on ZDV (or d4T) plus 3TC plus IDV.]