Although Chlamydia trachomatis is usually susceptible to the most frequently applied chemotherapeutic agents in non-gonococcal urethritis in men and its counterpart in women--i.e., tetracycline and erythromycin--the clinical results are less satisfying than those in comparable gonococcal infections. Therefore, potent therapeutical alternatives are urgently called for. From this point of view, we investigated the in vitro susceptibility of 35 recent isolates of genital Chlamydia trachomatis from Munich to the new quinolones ciprofloxacin and ofloxacin. With both chemotherapeutics, the highest minimum inhibitory concentration amounted to 2.0 micrograms/ml; 1.0 micrograms/ml of ofloxacin inhibited 97% of the strains, ciprofloxacin but 57%. The highest minimum bactericidal concentration was 6.0 micrograms/ml. 5.0 micrograms/ml of ciprofloxacin killed all infectious particles in 91% of the strains, the corresponding figure for ofloxacin read 74%. Thus both quinolones of the second generation proved more or less equally effective in vitro. In view of the limited clinical experience gained so far with regard to these chemotherapeutic agents, we suggest further clinical study in this matter.