This study will investigate the dose proportionality between single doses of 50mg and 100mg of GSK1349572 and will compare steady-state plasma PK, safety and tolerability of GSK1349572 50 mg every 24h (q24h) with and without efavirenz 600 mg q24h. Approximately 12 subjects will receive a single dose of GSK1349572 100 mg (Treatment A) in Period 1 followed by a washout of greater than or equal to 6 days. In Period 2 subjects will receive GSK1349572 50mg q24h for 5 days (Treatment B). Subjects will then be administered GSK1349572 50mg q24h in the morning in combination with EFV 600 mg q24h (Treatment C) in the evening for 14 days in Period 3. There will be no washout between Periods 2 and 3. Safety evaluations and serial PK samples for GSK1349572 will be collected during each treatment period. A follow-up visit will occur 7-14 days after the last dose of study drug.