Objective: To quantify unbound indinavir concentrations and characterize indinavir plasma protein binding in HIV-infected adults. Design: Pharmacokinetic study in antiretroviral-naive, HIV-infected persons with CD4 T lymphocytes > 100 x 106 cells/l and HIV-RNA in plasma > 5000 copies/ml at baseline who were participating in an open-label study of zidovudine, lamivudine and indinavir therapy. Methods: Eight men underwent 8 h intensive pharmacokinetic studies for indinavir on two occasions 6 months apart. Unbound indinavir was separated by ultra-filtration, and unbound and total concentrations were quantified by a validated high-performance liquid chromatography method. Results: Overall indinavir protein binding was 61 ± 6%, with a range among the profiles of 54 to 70%. Indinavir binding was higher at the 8 h post-dose concentration compared with the 1 h post-dose concentration (66 versus 57%, P= 0.0006). Conclusions: The mean 61% protein binding for indinavir in these HIV-infected persons is similar to the in vitro report of 60%. However, the fraction bound was concentration-dependent, and considerable variability in binding was present among patients. Quantification of unbound protease inhibitor concentrations opens new avenues of research to advance our understanding of the pharmacologically-relevant moieties of antiretroviral agents and thereby the pharmacotherapy of HIV infection. (C) 2000 Lippincott Williams and Wilkins.