The aim of this study was to evaluate the use of a hyperimmunized plasma containing high concentration of antibodies against Clostridium difficile, Clostridium perfringens and Salmonella sp in a referral population of equine colitis cases. A prospective, blinded clinical trial was undertaken. Horses were enrolled if they were over 1 year old, duration of diarrhea at presentation was less than 72 hours, they had not received equine plasma within the last 3 months and the serum total protein was greater than 4mg/dl. Horses were randomized to receive hyperimmunized plasma, control plasma (collected from non-immunized horses) or no plasma therapy. Clinical parameters were recorded and a fecal score (2 -14) assigned (every 6 hours) based upon diarrhea frequency, volume and consistency, for a total of 72 hours. A score less than 5 was considered normal. Fecal consistency was observed until resolution, discharge or death. Complete blood counts and biochemical profiles were collected at admission, 24 and 72 hours and at admission, 24 hours and 48 hours respectively. Forty two horses were enrolled and 38 horses completed the study. At study admission clinical and clinicopathological parameters, other than fecal frequency score were comparable between the groups. Fecal frequency score was significantly different between the treatment groups (p=0.003). The mean duration of diarrhea was 40.7±9.8 hours (mean ±SEM), 119.2±56.1 hours and 72.0±24.5 hours for the hyperimmunized plasma, normal plasma and control groups respectively. This data confirms the hyperimmunized plasma used in this study decreased the time to resolution of diarrhea. / Master of Science