In vitro studies of lactose hydrolysis in milk with 20-125 neutral lactase units (NLUs) carried out at 38.0 degrees C for 15 min with a beta-galactosidase derived from Kluyveromyces lactis (Lactaid, Lactaid Inc, Pleasantville, NJ) resulted in 85-95% of the hydrolysis observed with standard incubation conditions (24 h at 4-5 degrees C with 1000 NLU/L). Thirty-three lactose-maldigesting Guatemalan subjects, 16 children and 17 adults, were challenged with oral doses of lactose in milk (children aged less than 12 mo, 2 g/kg body wt; children aged 12-24 mo, 15 g/kg body wt; older children and adults, 18 g/kg body wt) preincubated for 20 min at 38 +/- 0.5 degrees C with 50-125 NLU Lactaid. Under these conditions the subjects consumed milk without presenting any signs of intolerance. Furthermore, their breath-hydrogen excretion showed a 91-93% reduction when compared with a similar load of milk containing nonhydrolyzed lactose.