OBJECTIVE: Previous studies have shown that selected strains of Lactobacillus have the capacity to antagonize rotavirus-induced diarrhea. However, only a few reports have documented their efficacy against nonrotavirus diarrhea. This study involved an experimental investigation and a clinical trial of the antisecretory activity of Lactobacillus acidophilus strain LB in the context of nonrotavirus diarrhea. METHODS: The activity of a culture of L. acidophilus LB or of the lyophilized, heat-killed L. acidophilus LB bacteria plus their spent culture medium was tested in inhibiting the formation of fluid-formed domes in cultured human intestinal Caco-2/TC7 cell monolayers infected with diarrheagenic, diffusely adhering Afa/Dr Escherichia coli C1845 bacteria. A randomized, double-blind, placebo-controlled clinical trial of male or female children who were 10 months of age and presented with nonrotavirus, well-established diarrhea was conducted to evaluate the therapeutic efficacy of a pharmaceutical preparation that contains 10 billion heat-killed L. acidophilus LB plus 160 mg of spent culture medium. RESULTS: Infection of the cells with C1845 bacteria that were treated with L. acidophilus LB culture or the lyophilized, heat-killed L. acidophilus LB bacteria plus their culture medium produced a dosage-dependent decrease in the number of fluid-formed domes as compared with cells that were infected with untreated C1845 bacteria. The clinical results show that in selected and controlled homogeneous groups of children with well-established, nonrotavirus diarrhea, adding lyophilized, heat-killed L. acidophilus LB bacteria plus their culture medium to a solution of oral rehydration solution shortened by 1 day the recovery time (ie, the time until the first normal stool was passed) as compared with children who received placebo oral rehydration solution. CONCLUSIONS: Heat-killed L. acidophilus LB plus its culture medium antagonizes the C1845-induced increase in paracellular permeability in intestinal Caco-2/TC7 cells and produces a clinically significant benefit in the management of children with nonrotavirus, well-established diarrhea.