Sixty-four men with non-gonococcal urethritis (NGU), seven with gonococcal urethritis (GU), and 30 who had no symptoms or signs of urethritis were studied. Chlamydia trachomatis was isolated from urethral specimens taken from 22% of the men with NGU, and 18% with GU, but not from those who did not have urethritis even though 20 (67%) of them had a history of NGU, GU, or both. The chlamydial isolation rate for men having NGU for the first time was 30%. Ureaplasma urealyticum was isolated from 42% of the men with NGU, 43% of men with GU, and 27% of those without urethritis.In addition to aerobes anaerobes were isolated frequently from men whether or not they had urethritis, the most common being anaerobic Corynebacteria, peptococci, and micro-organisms of the Bacteroides-Fusobacteria group. There was no appreciable difference in the overall isolation of anaerobes from men with NGU (89%) or from those without disease (80%). The rate of isolation of a Gram-negative anaerobic bacillus from men with NGU (50%) was, however, strikingly different to that from men with GU (14%) or from those without disease (13%). Furthermore, this bacillus was recovered from 28 (56%) of 50 men with NGU who were considered chlamydia-negative and from 19 (61%) of 31 men who were both chlamydia-negative and ureaplasma-negative. The effect of antibiotic treatment was evaluated in a few patients only, so that although those from whom the Gram-negative bacillus was eliminated by tetracycline recovered clinically there is a need for a prospective therapeutic trial to evaluate further the importance of the bacillus.