The number of intracytoplasmic inclusions of Chlamydia trachomatis produced in McCoy cell monolayer cultures infected with a constant inoculum of a recently isolated genital strain was compared in cultures of untreated replicating cells and in monolayers which had been incubated in the presence of cortisol at initial extracellular concentrations between 0.0001 and 100 microgram/ml. The effect of adding cortisol was dependent on its concentration, on the time of addition to the tissue culture medium, and on the initial number of McCoy cells seeded to form the monolayer. When a concentration of 1.0 microgram/ml was added at the time of infection with C. trachomatis, the number of inclusions detectable after a further 48 h of incubation was increased by 1.84-fold over those detected in untreated cells. The mean size of inclusions and the ease of their recognition in McCoy cell cultures was also increased by this procedure.