Thirty-five recent clinical isolates of Chlamydia trachomatis were subcultured and subjected to antimicrobial susceptibility testing with tetracycline and erythromycin. Detection of typical chlamydial inclusion bodies and elementary bodies was based on the use of fluorescence-labelled monoclonal antibodies. Minimum inhibitory concentration being defined as the lowest concentration suppressing all inclusion body formation and minimum bactericidal concentration as the lowest concentration preventing all detectable chlamydial growth, both these parameters were studied. With tetracycline the minimum inhibitory concentrations ranged from 0.03 to 0.08 microgram/ml, with erythromycin from 0.04 to 0.2 microgram/ml. The corresponding data for the minimum bactericidal concentrations were less than 0.2 to 1.0 and 0.2 to 2.0 respectively. Thus, at present, there still seems to be no major resistance problem with genital Chlamydia trachomatis isolates in the Federal Republic of Germany.