A new cell culture method was developed for the diagnostic isolation of Chlamydia trachomatis using McCoy cells pretreated with mitomycin-C. This drug acts on the cell by producing cross links between strands of DNA and results in the production of large, flat cells, which are similar to irradiated cells, and large chlamydial inclusions. In tests using a laboratory isolate of Chlamydia trachomatis, there were significantly more inclusions produced in mitomycin-treated monolayers than in cycloheximide-treated monolayers. Using clinical specimens, significantly more isolates were obtained in mitomycin-treated cells than in cycloheximide-treated cells. The new cell culture technique therefore offers a cell culture method which has the advantages of producing large inclusions similar to irradiated cells but with the convenience of drug treatment.