Atazanavir-d is intended for use as an internal standard for the quantification of atazanavir by GC- or LC-MS. Atazanavir is an azapeptide inhibitor of HIV-1 protease (K = 2.66 nM). It has antiviral activity against a variety of HIV-1 strains in several cell types with EC values ranging from 2.62 to 5.28 nM. Atazanavir exhibits a minor synergistic effect when used in combination with the reverse transcriptase inhibitor zidovudine in HIV-1-infected human peripheral blood mononuclear cells (PBMCs) and an additive effect when used in combination with several additional reverse transcriptase or HIV-1 protease inhibitors. Atazanavir also inhibits UDP-glucuronyltransferase 1A1 (UGT1A1), which is involved in bilirubin clearance. Formulations containing atazanavir have been used in combination therapy for the treatment of HIV-1 infection.