To elucidate whether Chlamydia trachomatis and C. pneumoniae infections occur to a significant extent in monocytes-macrophages, the human monocytic cell line, U-937, was infected with C. trachomatis L2 or C. pneumoniae TW-183. Chlamydial DNA and genus-specific antigens of the lipopolysaccharides (LPS) in epitopes of the chlamydial cell wall were detected from C. trachomatis L2-inoculated monocytes over a period of 150 days after inoculation and from the C. pneumoniae TW-183-inoculated cells during a period of 14 days. C. trachomatis-infected U-937 cells expressed significantly lower levels of CD4+, CD45RA+, CD11b+ and CD33+ cells, determined by flow cytometry, than control uninoculated cells on the seventh day after inoculation and they expressed a slightly increased level of CD4+ cells and lower levels of CD45RA+ and CD11b+ cells on the 14th day after inoculation. C. pneumoniae-infected U-937 cells expressed significantly lower levels of CD4+, CD45RA+, CD11b+ and CD33+ cells than controls on the seventh day after inoculation and an increased level of CD4+ and a lower level of CD45RA+ cells on the 14th day after inoculation. Unlike infection with C. trachomatis L2 strain, chronic persistent infection with C. pneumoniae appears not to occur in monocytes-macrophages.