Previous studies have suggested that monocytes may play a role in the dissemination ofChlamydia trachomatis, and in establishment of persistent infection with this bacterium. Infection of cultured human peripheral blood monocytes withC. trachomatisserovar K produced persistent, non-productive infection. Transmission electron microscopy of such infected cultures revealed single or multipleChlamydiain monocyte inclusions over a culture period of 10 days. Those inclusions were aberrant, and normal reticulate bodies within the inclusions were not observed. Immunoelectron microscopy showed the chlamydial major outer membrane protein and lipopolysaccharide to be associated with the bacterial plasma membrane. Lipopolysaccharide was also identified in the monocyte cytoplasm. Molecular analyses of primary chlamydial rRNA transcripts demonstrated that the organism is viable and metabolically active within monocyte inclusions. However, attempts to overcome chlamydial growth arrest by incubation ofChlamydia-infected monocytes with tryptophan, and antibodies against alpha interferon, gamma interferon, or tumor necrosis factor, were all ineffective, suggesting that known mechanisms of growth inhibition do not hold in human monocytes. These observations indicate that infection of human peripheral blood monocytes withC. trachomatismay be involved in the genesis/maintenance of extra-urogenital inflammation, since non-culturable, metabolically active bacteria persist in those cells.