Two established cell lines, H 292 and HEp-2, originating from the human respiratory tract, were found to be significantly more efficient and practical than the currently used HeLa 229 cells for growth of Chlamydia pneumoniae. Six strains of C. pneunoniae recently isolated from patients with respiratory ailments were used as test cultures. The H 292 and HEp-2 cells yielded much higher inclusion counts for all the test strains than did HeLa 229 cells. When they were compared with each other, H 292 cells yielded more inclusions than did HEp-2 cells, and the differences were statistically significant in 10 of 18 test sets. A simple system with these two cell lines appeared to be very efficient for culturing C. pneumoniae. It does not require treatment of tissue cells with DEAE-dextran before infection, and it may eliminate the need for serial subpassages of specimens to increase culture sensitivity. Monolayers of these cells remained intact and viable in the Chlamydia growth medium so that reinfection could take place, resulting in greatly increased inclusion counts for specimens containing few infectious units. This system may make it more practical for laboratories to culture for C. pneumoniae for treatment of infections and outbreak intervention and will facilitate studies on this recently recognized pathogen. * Corresponding author. were isolated from patients with respiratory ailments in a military training camp in Norway during an adenovirus outbreak (1) and were kindly provided by Bjorn P. Berdal, University of Tromso, Tromso, Norway. All strains were isolated in HeLa 229 cells. Strain CWL 029 was used as antigen for production of monoclonal antibodies for identification of C. pneumoniae in tissue cells. The other six isolates in the third to fifth subpassages in HeLa 229 cells were used as inocula to test the selected cell lines. Serotype strains of C. trachomatis, originally from S. P. Wang of the University of Washington, Seattle, and C. psittaci PI-1 and D44 were from our stock cultures. Cell lines. All cell lines were obtained from the Cell Culture Section at the Centers for Disease Control and were passaged by standard tissue culture techniques. The following five cell lines derived from human respiratory tissues were tested in reference to HeLa 229 from cervix carcinoma:

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