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The growth characteristics of Chlamydia psittaci serotypes 1 and 2 (ovine and bovine origin) were studed in mouse L cells. Formation of inclusions and yield of infectious progeny for serotype 1 were maximal when host cells were treated with cycloheximide and the pH in the cell culture medium was 7.2-7.4. The number of cells that contained inclusions and the infectivity yield for serotype 2 were maximal when the pH was 6.6-7.0. Treatments with diethylaminoethyl dextran and cycloheximide increased inclusion formation in serotype 2 but decreased the yield per infected cell. For both serotypes, centrifugation of the chlamydiae onto monolayers infected 50-1,200 times the number of cells infected with use of stationary adsorption. An enumeration method for L-cell infectious units was developed that gave higher titers than the chicken embryo 50% lethal dose after yolk sac inoculation.