The aim of the present study was to determine whether a fermented milk containing Lactobacillus casei and Lactobacillus acidophilus strains isolated from human subjects (CFM) and developed by CERELA (Centro de referencia para Lactobacilos) is better tolerated than regular milk (RM) by subjects with lactase deficiency (< 1 unidad) and lactose intolerance. We studied the digestion of the lactose present in the two milk preparations which were administered to 18 healthy subjects with lactase deficiency and 12 control subjects. Each subject ingested in random sequence on different days, 480 ml CFM, 480 ml RM and 240 ml CFM and 240 ml RM. Breath hydrogen (H2) test was used to measure lactose absorption and orocecal transit time (OCTT). The peak H2 ppm results after 480 ml of CFM were ingested, reached 19.5 +/- 12.1 ppm. The same reading when 480 ml of RM were ingested was 52.6 +/- 31.9 ppm (p < 0.008). The results also showed that 480 ml of CFM delayed OCTT median 111.0 +/- 6.78 min versus 54.0 +/- 5.09 min of RM (p < 0.001) and reduced the development of symptoms (p < 0.08) such as bloating (p < 0.05), borborygmi (p < 0.025) diarrhea (p < 0.05) and abdominal colics (p < 0.05). In spite of the small number of cases studied, it seems justified to conclude that CERELA fermented milk significantly influences lactose digestion and minor development of symptoms in lactase-deficient subjects and lengthens significantly the orocecal transit time compared with regular milk.