An endothelial cell monolayer grown on a microporous membrane coated with basement membrane protein matrix was used to study translocation of yersinia-infected human polymorphonuclear leukocytes (PMNs). PMNs infected with one to eight bacteria were able to translocate living yersiniae from the upper chamber to the chemoattractant-containing lower chamber. This process may contribute to extravasation and dissemination of yersiniae in the infected host.