Sixteen strains of Lactobacillus isolated from humans, mice, and food products were screened for their capacity to associate with Peyer's patches in mice. In preliminary experiments, in vitro binding to tissue pieces was assessed by scanning electron microscopy, and it was demonstrated qualitatively that 5 of the 16 strains showed some affinity for the Peyer's patches, irrespective of their association with the nonlymphoid intestinal tissue. Lactobacillus fermentum KLD was selected for further study, since, in addition to its intrinsically high adhesion rate, this organism was found to exhibit a preferential binding to the follicle-associated epithelium of the Peyer's patches compared with its level of binding to the mucus-secreting regions of the small intestine. Quantitative assessment of scanning electron micrographs of tissue sections which had been incubated with L. fermentum KLD or a nonbinding control strain, Lactobacillus delbruckii subsp. bulgaricus, supported these observations, since a marked difference in adhesion was noted (P < 0.05). This preferential association of strain KLD with the Peyer's patches was also confirmed with radiolabeled lactobacilli incubated with intestinal tissue in the in vitro adhesion assay. Direct recovery of L. fermentum KLD from washed tissue following oral dosing of mice revealed a distinct association (P < 0.05) between this organism and the Peyer's patch tissue. In contrast, L. delbruckii subsp. bulgaricus showed negligible binding to both tissue types in both in vitro and in vivo adhesion assays. It was concluded that L. fermentum KLD bound preferentially to Peyer's patches of BALB/c mice.