Certain microbes like yersinia, salmonella, shigella, campylobacter, chlamydia, and possibly gonococcus can trigger reactive arthritis especially in patients of the HLA-B27 type. In the present study we have used serological and culture methods to identify the probable triggering infection in 50 consecutive HLA-B27 positive patients diagnosed as having reactive arthritis. The two most common triggering agents thus identified were Yersinia enterocolitica (12 patients) and Chlamydia trachomatis (11 patients). In addition six patients had high antistreptolysin O titres and two high teichoic acid antibody titres suggesting group A streptococci and Staphylococcus aureus as triggering agents. In 13 patients no preceding infection could be identified. The identity of the infective agent seems to have very little effect on the clinical picture of the reactive arthritis - the only difference between the various aetiological groups in the present material was absence of fever in the patients with a preceding C. trachomatis infection, of whom only one out of 11 had a temperature greater than or equal to 38 degrees C, whereas 13 of 16 patients with a preceding enterobacterial, and five of the eight patients with a streptococcal or staphylococcal infection had raised temperatures.