In this paper, we describe the development and use of enzymatic assays to determine intracellular lamivudine triphosphate (3TCTP) and carbovir triphosphate (CBVTP) concentrations in peripheral blood mononuclear cells (PBMCs) from human immunodeficiency virus (HIV)-infected patients. The assays involve inhibition of HIV reverse transcriptase (RT), which normally incorporates radiolabeled deoxynucleoside triphosphates into a synthetic template primer. For the 3TCTP assay, a preincubation procedure was added whereby 3TCTP becomes incorporated before [(3)H]dCTP. At a 1:400 template primer dilution, control product formation was reduced by 88.0% with 0.8 pmol of 3TCTP. Standard 3TCTP inhibition curves were performed using this procedure. For the CBVTP assay, 0.1 pmol of CBVTP inhibited control product formation with and without the use of a preincubation step, so inhibition curves were constructed using both procedures. However, reduced template primer stability with assays using preincubation steps led to a single-incubation procedure being adopted for future studies. The presence of PBMC extracts interfered with the 3TCTP assay. However, this was overcome by the addition of CuSO(4). PBMC extracts did not interfere with the CBVTP assay. Intracellular 3TCTP and CBVTP concentrations were determined in PBMCs from HIV-infected patients over 24 h or greater. Peak concentrations were obtained 6 to 8 h after dosing, and the half-lives of the anabolites suggested the possibility of once-daily dosing. These assays are currently being used for determination of 3TCTP and CBVTP concentrations in clinical studies.