Thirty-two lactobacilli were tested for ability to adhere to a human fetal intestinal epithelial cell line. By an in vitro system, two adherence mechanisms were found. One mechanism, requiring calcium in the adherence reaction, was nonspecific and allowed all lactobacilli tested to adhere. The other system, not requiring calcium, was found in four strains, all human Lactobacillus acidophilus isolates. Colonial morphology, serial broth passage, and exposure of cell crops to freezing or lyophilization did not affect adherence of Lactobacillus acidophilus. In vitro adherence, combined with subsequent in vivo studies, may provide a basis for screening candidate organisms for use in microbiotic supplements.