Crystalline lactulose (Laevolac Cristalli, CAS 4618-18-2), a pure form of the disaccharide widely employed in the therapy of complications of liver cirrhosis, was administered, after a pharmacological wash-out of 10 days and following a randomized design, to 10 cirrhotic patients for 30 days at the dosage of 60 g/d, while another 10 subjects with similar characteristics received no treatment. Besides the parameters usually monitored for the evaluation of liver function and state of hepatic encephalopathy, immunological patterns such as lymphocyte subpopulations CD3, CD4, CD8, CD16(NK), CD25 and antibacterial activity against Ty2 strain of Salmonella typhi were evaluated. At the end of the study (day 30) a significant decrease of blood ammonia was observed, as expected, only in the group treated with lactulose with respect to the control group, as well as a significant increase of CD16 and antibacterial activity (1/3); an enhanced level of CD25, although not significant, was also noticed in the treated group with respect to the control group. These findings seem to show an effect of activation on cell-mediated immune system depressed during liver cirrhosis, produced by lactulose. Further studies are needed to confirm these data and to clarify the possible mechanisms involved.