Tenofovir disoproxil fumarate (TDF) 9-[(R)-2-[[bis [[isopropoxycarbonyl] oxy] methoxy] phosphonyl] methoxy] popyl] adenine fumarate [Figure 1] is a nucleotide analog reverse transcriptase inhibitor (NRTI) and is used for treating HIV infection in adults, in combination with other antiretroviral agents. Lamivudine (LAMI) 4-amino-1-[(2R,5S)-2-(hydroxyl methyl)-1,3-oxathiolan-5- yl]-1,2-dihydro pyrimidin-2-one [Figure 2] is an NRTI used in the treatment of HIV infection and chronic hepatitis B virus (HBV). Efavirenz (EFV) [(4S)-6-chloro-4-(cyclopropylethynyl)-1,4-dihydro-4-(trifluoromethyl)-2H-3,1-benzoxazin-2-one] [Figure 3] is a non-nucleotide reverse transcriptase inhibitor (NNRTI) used in the combination treatment of HIV infection (AIDS).[1�C6] The combination of TDF, LAMI, and EFV (300 mg TDF, 300 mg LAMI, and 600 mg EFV) was approved in July 2008 by Central Drug Standard Control Organization (CDSCO) and tentatively approved by US Food and Drug Administration (USFDA) on 9 March 2009 for the treatment of HIV infection in adults.[7,8] Literature survey reveals that TDF is estimated individually by titrimetric, UV, reverse phase-high performance liquid chromatography (RP-HPLC) in tablet formulation and by RP-HPLC methods in human plasma.[9�C14] Few UV, RP-HPLC, high performance thin layer chromatography (HPTLC), and liquid chromatography with tandem mass spectrometry (LC/MS/MS) methods have been reported for simultaneous estimation of emtricitabine and TDF in pharmaceutical formulation.[15�C18] Similarly, estimation of LAMI by titrimetric, UV, HPTLC, RP-HPLC methods in tablet formulation and by RP-HPLC method in human plasma has been reported.[19�C23] Few UV, RP-HPLC, HPTLC, and LC/MS/MS methods have been reported for the simultaneous estimation of LAMI with nevirapine, stavudine, and zidovudine.[24�C28] Also, for the estimation of EFV, UV, HPTLC, and isocratic HPLC methods in tablet formulation and HPLC method in human plasma have been reported.[25�C33] Also, RP-HPLC method is reported for the simultaneous estimation of EFV with TDF and emtricitabine.[34] Only one method, that is UV spectrophotometric method, is used for the simultaneous estimation of this combination of TDF, LAMI, and EFV in their combined tablet dosage form.[35] The purpose of this study was to develop a simple, rapid, precise, and accurate RP-HPLC method for the simultaneous estimation of these drugs in combined tablet dosage form. Figure 1 Chemical structure of tenofovir disoproxil fumarate Figure 2 Chemical structure of lamivudine Figure 3 Chemical structure of efavirenz MATERIALS AND METHODS Instrumentation Liquid chromatographic Shimadzu (LC-2010CHT) system was manufactured by Shimadzu, Kyoto, Japan, and is equipped with auto-sampler, UV and Photodiode Array (PDA) detector, and Rheodyne injector with 20 ��l loop volume. Weighing was done on a Digital Micro Balance an Acculab ALC 210.