The absolute bioavailability of clarithromycin, a new macrolide antimicrobial agent, was assessed in a three-way, randomized, single-dose, crossover study conducted with 22 healthy volunteers, 19 of whom provided analyzable study data. The bioavailability parameters of two 250-mg oral tablet formulations were calculated with reference to an identical dose administered by intravenous infusion of the lactobionate salt. After adjustment for formulation potency, the mean absolute bioavailabilities of the two oral formulations were 52 and 55%, on the basis of the appearance of parent compound in the systemic circulation. Metabolite peak concentration and area under the plasma concentration-time curve data after oral dosing were generally greater than those after intravenous infusion, suggesting that marked first-pass metabolism of clarithromycin occurs after oral administration. Pharmacokinetic analysis of the parent drug and the active 14-hydroxy metabolite data suggests complete (or nearly complete) absorption of the drug after oral administration.