A total of 19 pathogenic bacterial species was isolated from uterine swabs of 498 out of 1539 mares over 4 years. The swabs were taken by 5 veterinary clinicians using 2 different techniques. Bacterial contamination during swabbing was minimized by scrupulous attention to cleansing of the external genitalia and the perineal area, and in the handling of the culture specimen. The most prevalent organisms isolated were beta-haemolytic streptococcus (39%), Escherichia coli (27%) and Klebsiella pneumoniae (7%). Interpretation of microbiological findings correlated well with clinical findings when number of organisms isolated and endometrial cytology were considered. The use of a bacterial transport medium combined with sophisticated culture methods reflects a more accurate picture of the uterine microflora than can be obtained by previous techniques. Streptococci isolated were uniformly sensitive to penicillins. The sensitivity of E. coli and K. pneumoniae towards chloramphenicol, gentamicin and polymyxin was nearly 100%. The selection of an appropriate antibacterial agent depends upon sensitivity, pharmacological action, genital tract status and cost. This study shows that a Gram stain of uterine cytology can be used to diagnose quickly and select an appropriate antibiotic for treatment prior to culture results if sufficient numbers of organisms are present.