Neisseria gonorrhoeae 188, type 1, was cultured with monolayers of Rhesus monkey kidney (RE2) cells. Attachment of gonococci to RE2 cells occurred from about 2 h after inoculation and appeared maximal by 6 h when stained with crystal violet solution and examined under a light microscope. Preferential attachment of gonococci to the perinuclear region of tissue cells could not be demonstrated. Enumeration of the colony-forming units revealed that gonococci became rapidly attached to RE2 cells, increased in number, and after 8 h gradually decreased. The number of cell-free gonococci (not bound to tissue cells) remained constant for the first few hours and then changed in parallel with the cell-bound bacterial number. Gonococci did not multiply in tissue-culture medium. When penicillin was added to the system at intervals, neither cell-free nor cell-bound gonococci were recovered. Electron microscopy of RE2 cells infected with N. gonorrhoeae revealed no intact intracellular organisms, only gonococci attached to tissue cells and imcompletely engulfed or partially digested gonococci. These observations indicate that gonococci only multiply extracellarly in the presence of RE2 cells. Furthermore, some gonococci were engulfed by RE2 cells, but appeared to be digested so rapidly that they could not multiply intracellularly.