Systematic modifications of HIV protease inhibitor (2R,3S,4S)-4-[[(benzyloxycarbonyl)-L-valyl]-amino]-3-hydroxy-2-[(4- methoxybenzyl)amino]-5-phenylpentanoyl)-L-valine 2-(aminomethyl)- benzimidazole amide led to a novel series of inhibitors with shortened, modified carboxy terminus. Their synthesis, in vitro enzyme inhibitory data, and antiviral activities are reported. Of particular interest are derivatives featuring the (1S,2R)-1-amino-2-hydroxyindan moiety at the P2'-position since some of them exhibit substantial oral bioavailability in mice. The influence of aqueous solubility and structural parameters on the oral resorption of the inhibitors is discussed. Optimum enhancement of oral bioavailability was observed with L-tert-leucine in P2-position, resulting in the discovery of (2R,3S,4S)-4-[[(benzyloxycarbonyl)-L-tert-leucyl]- amino]-3-hydroxy-2-[(4-methoxybenzyl)amino]-5-phenylpentanoic acid (1S,2R)-1-amino-2-hydroxyindan amide which combines high antiviral activity (IC50 = 250 nM) with a good pharmacokinetic profile (AUC = 82.5 microM.h at a dose of 125 mg/kg po in mice).