The new oral 200-mg rifamycin SV MMX modified-release tablets, designed to deliver rifamycin SV directly into the colonic lumen, offer considerable advantages over the existing immediate-release antidiarrheic formulations. In two pharmacokinetics studies of healthy volunteers, the absorption, urinary excretion, and fecal elimination of rifamycin SV after single- and multiple-dose regimens of the new formulation were investigated. Concentrations in plasma of >2 ng/ml were infrequently and randomly quantifiable after single and multiple oral doses. The systemic exposure to rifamycin SV after single and multiple oral doses of MMX tablets under fasting and fed conditions or following a four-times-a-day (q.i.d.) or a twice-a-day (b.i.d.) regimen could be considered negligible. With both oral regimens, the drug was confirmed to be very poorly absorbable systemically. The amount of systemically absorbed antibiotic excreted by the renal route is far lower than 0.01% of the administered dose after both the single- and multiple-dose regimens. The absolute bioavailability, calculated as the mean percent ratio between total urinary excretion amounts (ΣXu) after a single intravenous injection and after a single oral dose under fasting conditions, was 0.0410±0.0617. The total elimination of the unchanged rifamycin SV with feces was 87% of the administered oral dose. No significant effect of rifamycin SV on vital signs, electrocardiograms, or laboratory parameters was observed.