The objectives of this trial are in Part 1 to compare the rate and extent of absorption of a single oral dose of 600 mg darunavir (DRV) administered as the suspension formulation F051 (under fed and fasted conditions) to that of the commercial tablet formulation F016 (under fed conditions), in the presence of low-dose ritonavir in healthy volunteers; and to compare the rate and extent of absorption of a single oral dose of 600 mg DRV administered as the suspension formulation F051 under fed conditions to that under fasted conditions, in the presence of low-dose ritonavir in healthy volunteers. In Part 2 we want to assess multiple dose pharmacokinetics of DRV administered as the suspension formulation F051 (under fed or fasted conditions, based on the results of Part 1), in the presence of low-dose ritonavir in healthy volunteers. The short-term safety and tolerability of DRV will be evaluated following administration of 3 single oral doses of 600 mg (formulated as suspension and as tablet) and following administration of multiple oral doses (formulated as suspension), all in presence of low-dose ritonavir in healthy volunteers Part 1: during 3 sessions volunteers will orally receive 3 single doses of 600 mg darunavir (DRV): 2 tablets of 300 mg and 6 ml of suspension under fed conditions and 6 mL of suspension (100 mg/mL) under fasted conditions on Day 3. Part 2: subjects will receive DRV suspension, twice daily from Day 1 to 6, with an additional morning dose on Day 7. In Part I rtv 100 mg will be coadministered twice daily from Day 1 to Day 5 and in Part 2 from Day 1 to Day 9.