A passive haemagglutination test (PHT) which has been developed for the detection of antibodies to the contagious equine metritis organism (CEMO) in serum is described. Samples from each of 30 mares with metritis were positive with titres in the range 256 to 4096. Samples from each of 239 clinically normal mares and 30 colts and fillies believed not to have been exposed to CEMO were negative with titres of less than 256, the majority of samples (97 per cent) showing a titre of 32 or less.