Two assay systems for fibrinogen-fibrin degradation products (FDP) in canine serum, the glutaraldehyde erythrocyte hemagglutination-inhibition immunoassay (GEHII) and the staphylococcal clumping test (SCT), were compared for reactivity with the plasmin digests of canine fibrinogen and with sera obtained from dogs suspected of having circulating FDP, The GEHII reacted well with early and late plasmin digests of fibrinogen. With early-stage fibrinogen digests, the SCT was more reactive than was GEHII. However, after the appearance of D and E fragments in the digests, SCT reactivity decreased progressively during plasminolysis. The sensitivities of GEHII and SCT for assaying FDP were 0.5 and 0.125 microgram of fibrinogen equivalent/ml, respectively. Serum FDP values for dogs with desease were comparable, with values ranging from 1 to 122 micrograms/ml by GEHII and 8 to 64 micrograms/ml by SCT. One dog with fulminating disseminated intravascular coagulation had a serum FDP value of 128 micrograms/ml by SCT and 179 micrograms/ml by GEHII. Howeever, normal canine sera had values by SCT of 4 to 16 micrograms of fibrinogen equivalent/ml, whereas normal values by GEHII were 0.5 to 2 micrograms/ml. Serum FDP concentrations were also determined for healthy and diseased dogs by latex-agglutination test. The dog with fulminating disseminqated intravascular coagulation had a FDP value of greater than 40 micrograms/ml, and only 1 other diseased dog had greater than 10, but less than 40 micrograms of fibrinogen equivalent/ml. All other dogs were considered negative, with FDP values of less than 10 micrograms/ml.