Three methods of isolation and identification of Chlamydia trachomatis inclusions in cell monolayers were compared: Giemsa staining followed by darkfield microscopy of cycloheximide treated Buffalo green monkey kidney monolayers (BGM), fluorescent monoclonal antibody staining of cycloheximide treated BGM and of HeLa 229 monolayers. In an unselected group of 895 patients with suspected chlamydial infections including contacts of symptomatic patients, significantly more positive chlamydia isolates were detected with immunofluorescent monoclonal anti-body staining of BGM and HeLa 229 than with Giemsa staining followed by darkfield microscopy on BGM. However, there was no disparity between the 2 staining methods with specimens from 156 patients with untreated non-gonococcal urethritis. In all chlamydia positive specimens, significantly more inclusions were seen after BGM-immunofluorescence than after HeLa-immunofluorescence or BGM-Giemsa. The extra cost associated with immunofluorescence staining was justified by the time saved in scanning.