Sexually transmitted diseases (STDs) may recognize multiple etiological agents. Among them, Chlamydia trachomatis (Ct) and herpes simplex virus type 2 (HSV-2) cause symptomatic, subclinical and asymptomatic infections of the urogenital tract which can lead to serious sequelae. In the present study the coinfection and superinfection by Ct and HSV-2 in epithelial cultured cells from human cervix (HeLa 229) are described. A double infection, followed by the intracellular synthesis of chlamydial and viral antigens, was established. Both synergistic and interfering phenomena were recorded: viral antigen synthesis resulted increased whereas Ct inclusion bodies were produced to a lower extent.