The purpose of this study is to assess the pharmacokinetics of plasma lopinavir/ritonavir following administration to male and female HIV−negative volunteers of 400/100mg, 200/150mg and 200/50mg lopinavir/ritonavir twice daily. Each dosing phase will last for 7 days and each phase will be separated by a 7-day wash-out period. Pharmacokinetic evaluations will be made over a 12-hour interval at the end of each dosing phase.