108 male patients with non-gonococcal urethritis have been treated with minocycline given as two 100 mg. tablets initially, followed by one tablet twice a day over a period of 6 days (13 tablets). Of 96 patients followed, re-treatment for non-gonococcal urethritis within 3 months was undertaken in ten (10.4 per cent.). The findings are compared with those previously reported in seven series involving six other tetracyclines and with nine other treatment regimens. All of the tetracyclines proved superior to other agents and the best results combined with simplicity of administration were obtained with minocycline. Two patients complained of soreness of the mouth after 1 week but no other side-effects were reported. Minocycline thus gives excellent results in the treatment of non-gonococcal urethritis. In the dosage used it was also successful against gonorrhoea in seven patients treated. It is therefore particularly useful in cases in which gonorrhoea is suspected but not found in the smears or in which the smears cannot be read immediately.