ABSTRACT. To evaluate the response of the small intestinal mucosa to Saccharomyces boulardii (S.b.), a yeast widely used in some countries as an adjuvant drug with oral antimicrobial therapy, seven healthy adult volunteers were treated with high doses of lyophilized S.b. (250 mg four times per day) for 2 wk. A peroral jejunal suction biopsy was performed on days 0 and 15 of the study. Compared to the initial biopsy, histological examination of the posttrial biopsy revealed no morphological alteration nor change in villus height or crypt depth. After treatment, the specific activity (per U protein) of sucrase, lactase, and maltase was, respectively, increased by 82% (p<0.05) 77% (p<0.05), and 75% (p<0.05) over the basal activity of the enzymes measured on day 0, whereas mucosal protein content remained unchanged. Similar findings were found in the jejunum of adult rats treated for 5 days with either viable or killed S.b. cells. The changes in total enzyme activity (per jejunal segment) paralleled the changes in specific enzyme activity. In vitro assays on freshly prepared suspensions of S.b. (6.0 × 108 viable cells/ ml) evidenced a high activity for sucrase (mean ± SE: 8 364 ± 1280 U · g · protein-1) but no maltase, neutral lactase, acid β-galactosidase, or aminopeptidase activity. To determine whether treatment with S.b. could influence the incorporation rate of neutral lactase into the brush border membrane, 14-day-old sucklings treated either with saline or with S.b. were given intraperitoneally a dose of 20/µCi D-[114C] glucosamine 3 hours before sacrifice. Neutral lactase was isolated on SDS-PAGE of purified BBM. The amount of lactase protein eluted from the gel slices was similar in treated rats (mean ± SE: 0.026 ± 0.003) and in controls (0.021 ± 0.005 mg protein/ml). Expressed per milligram of brush border membrane lactase protein, there was no significant difference in the incorporation rate of labelled glucosamine between treated rats (mean ± SE: 8167 ± 1622 dpm-mg protein−1) and controls (9602 ± 1803 dpm-mg protein−1). In conclusion, short-term oral treatment of human volunteers and rats with S.b. is associated with a marked increase in the activity of disaccharidases without morphological alteration of the intestinal mucosa. Our findings do not suggest an effect of S.b. on the incorporation rate of enzymes into the brush border membrane.