A two-way, open-label, crossover study in 12 subjects was undertaken to study the potential for azithromycin to alter the pharmacokinetics of nelfinavir and/or its active metabolite, M8. A secondary objective was to characterize any potential interaction that nelfinavir may have with azithromycin. During one dosing arm, subjects received a single 1200 mg oral dose of azithromycin. During the other, subjects received 11 days of nelfinavir 750 mg q8h with a single 1200 mg oral dose of azithromycin given concurrently with the Day 9 morning nelfinavir dose. Serum samples were collected after each azithromycin dose for 168 hours and after the Day 8 and 9 morning nelfinavir doses for 8 hours to characterize azithromycin, nelfinavir, and M8 pharmacokinetic parameters during both control and test periods. Both dosing regimens were well tolerated, with only mild to moderate GI side effects being the most frequently reported. Azithromycin was found to cause a statistically, though not clinically, significant decrease in nelfinavir and M8 exposures. In contrast, nelfinavir caused azithromycin Cmax and exposure (AUC) values to increase by > 100%. Inhibition of p-glycoprotein by nelfinavir may be responsible for this significant interaction. This increase in azithromycin exposure has the potential to increase clinical antibacterial efficacy without significantly increasing gastrointestinal side effects, though the impact on other systemic sites needs to be studied.