The authors conducted a randomized trial on 16 patients to evaluate intestinal aerobic microfloral changes after prolonged oral treatment with bacampicillin (b.) (16 g/die) or amoxicillin (a.) (2 g/die). The analysis showed a quantitative reduction of isolates in 6 patients: 2 patients were treated with b. while 4 with a. (Odd ratio, O.R. = 3). Mean values of CFU presented as well a more evident reduction in a.-treated patients. From the qualitative point of view, bacteria modifications occurred in one patient treated with b. and 3 with a, (O.R. = 4.2). Bacterial changes, although not statistically significant, were thus greater in patients treated with a. than b.