A trial comparing the efficacy of three different tetracyclines, each in two different dosage regimens, in the treatment of non-specific urethritis is described. There was a significant association between the retreatment rate and sexual intercourse. Age, race, duration of symptoms, previous infection, and indulgence in alcohol did not appear to influence the results whereas treatment of sexual contacts before resumption of sexual intercourse significantly reduced the retreatment rate. Single-dose treatment with doxycycline was shown to be ineffective. Treatment with doxycycline for seven days or triple tetracycline for seven days was less effective than triple tetracycline for 21 days or oxytetracyline for seven or 21 days, all of which gave the same success rate.