Chlamydia trachomatis (C.t.) cell culture represents a sensitive method for the diagnosis of chlamydial infection and the only one which makes it possible to determine the susceptibility of an isolate to antibiotics so that an appropriate drug can be selected for individual treatment. In 11 patients, affected by urethroprostatitis and suspected of treatment failure with standard drug regimens, either due to lack of compliance with therapy or antibiotic resistance, C.t. was isolated in McCoy cell culture from urethral swabs, after prostatic massage. The in vitro activity of azithromycin against these isolates and the in vivo efficacy of the drug in the patients treated with a single 1 g dose have been evaluated. All the C.t. strains tested were susceptible to the action of azithromycin (MIC range 0.125-1.0 microgram/ml). Bactericidal values were one dilution higher (MBC range 0.25-2.0 microgram/ml). These in vitro results are consistent with clinical observations as all the patients treated had negative culture at a 4-week follow-up visit.