After a single dose of cyclophosphamide (100 mg/kg) cell proliferation in the jejunum of the rat decreased within the first 24 h and returned to the initial level after 48 h. Under the influence of cyclophosphamide, an increased cell loss in germfree rats could be observed. Villus height and villus cell count tended to decrease. Changes in disaccharidase activity in mucosal scrapings with respect to protein and DNA content could not be demonstrated. An influence of presence or absence of bacterial flora could be observed.