Employed were the virus-neutralization test (VNT), the complement-fixation tests (CFT), and the immunodiffusion test (IT) in the study of serum samples of calves on a total of 25 farms that had records of virus diarrhea-mucosal disease (VD-MD) in the course of two years. Samples were also taken from calves in the initial phase of the same disease in the course of four months. The blood of experimentally infected calves waw likewise sampled for three months. It was found that the stage of infection could successfully be studied via all three serologic methods. Thus, for example, positive VNT and CFT results coupled with negative IT results spoke of the initial stage of the disease, and vice versa--negative CFT results and positive VNT and IT results gave evidence of the advanced stage of the infection.