Non-specific urethritis (NSU) is treated as an entity, yet it includes cases of urethral inflammation presumed to be related to the presence of probably several specific organisms. The effectiveness of the tetracycline group of antibiotics in comparison with other forms of treatment is not in doubt (Wilcox, 1972). In the present study we wish to remark upon the aetiology of NSU and to discover whether tetracycline treatment can assist us in understanding the nature of NSU, and also to comment on the best application of tetracycline therapy.NSU Sompolinsky, Harari, Solomon, Caspi, Krakowski, and Henig, 1973;Wentworth, Bonin, Holmes, Gutman, Wiesner, and Alexander, 1973). Yet the role of this agent, although probably at times a pathogen, is still questionable (Richmond, Hilton, and Clarke, 1972;Hilton, Richmond, Milne, Hindley, and Clarke, 1974;Oriel, Powis, Reeve, Miller, and Nicol, 1974).It may be that Chlamydia agent plays an 'opportunist' role, and acts both as pathogen and as 'potential pathogen' in the genito-urinary tract. Much of this work is relatively new and still continuing. A recent study on the presence of antibodies to Chlamydia in patients with urethritis, and in persons without urethritis, does not make the position any clearer (Reeve, Gerloff, Casper, Philip, Oriel, and Powis, 1974). Mycoplasma hominis (and to a much lesser extent M. fermentans and M. salivarius) and T-strain mycoplasma can be present in those with a healthy genito-urinary epithelium as well as in those in whom the epithelium is inflamed. It would appear from the evidence that the T-strain may be present in over 60 per cent. of cases of urethritis, and M. hominis in about 20 per cent. of cases.