Bovine viral diarrhea virus (BVDV) is associated with a range of economically important diseases of cattle including reproductive disorders and an acute fatal hemorrhagic disease. Neutralizing antibodies that bind to the E2 glycoprotein are important predictors of vaccinal immunity. Neutralization tests using the NADL strain of BVDV and five anti-E2 monoclonal antibodies showed one, Wb163, neutralized the NADL strain of BVDV in an unexpected manner. Its titer was 10,000 compared to <35 as reported previously. The present stock of NADL differed from that of the earlier study in that the amino acid at position 79 of E2 was Valine instead of Glutamic acid. MAb Wb163 may, however, recognize a less important neutralizing epitope than another mAb Wb166, because it was less cross reactive than mAb Wb166, had a neutralizing titer 50-fold lower than Wb166 and was of lower relative affinity than Wb166. Variations in the amino terminus of E2 will be discussed in the context of vaccinal immunity.