Background: The extent of preservation of clotting factors and incidence of transfusion reactions to noncommercial equine plasma is not documented.Hypothesis: Equine frozen plasma would retain its coagulation factor activity within the reference range and the incidence of transfusion reactions would be low.Animals: Ten plasma donor horses. Fifty clinically ill hospitalized horses receiving plasma were reviewed to determine the incidence of reactions.Methods: In vitro study and retrospective case review. Plasma was prepared by gravity sedimentation from whole blood refrigerated for 48 hours. The activities of factors VII through XII, antithrombin (AT), and Protein C were measured. Factor activities were compared for plasma samples obtained before blood collection (S0), after 48 hours of gravity sedimentation at 5 1C and after plasma separation (S1), and after 90 days of storage at À20 1C (S90). The medical records of 50 consecutive clinically ill horses receiving frozen plasma were reviewed to determine the incidence of transfusion reactions.Results: The combined effect of plasma harvest, gravity sedimentation, decantation, and freezing caused significant reductions in factors IX, (43% P 5 .0013), X, (33% P 5 .0001), XI, (48% P 5 .0008), AT, (10% P 5 .02), and Protein C (26% P 5 .0001). Activities for all factors analyzed, except factor X, remained within the reference ranges. Transfusion reactions were recorded for 5/50 horses.Conclusions and Clinical Relevance: Clotting factors, AT, and Protein C were well preserved. The incidence of reactions to frozen plasma was 10%.