Hybridoma technique has been used to obtain monoclonal antibodies to specific antigenic determinants on complex antigens. For example, myeloma cells, which are resistant to 8-azaguanine due to deficiency of hypoxanthine phosphoribosyl transferase and can not survive in media containing hypoxanthine, aminopterine, and thymidine CHAT), is hybridized to primed spleen cells from animal immunized with specific antigens. These cells can survive in HAT media, proliferate continuously, and secrete homologous antibody to specific antigens. The M variant of encephalomyocarditis (EMC) virus contains two stable variants, one diabetogenic CD variant) and the other nondiabetogenic CB variant). When the D variant of EMC virus was inoculated into SJL/J mice, hyperglycemia developed in over 90% of the animals, while none of the mice developed diabetes when inoculated with the B variant of EMC virus. However, the D and B variants are immunologically indistinguishable from each other by standard conventional method: Antibody made against the D variant neutralized the D and B variants to about the same degree. Similarly, antibody made against the B variant neutralized the Band D variant equally well. To distinguish antigenic specificity of these two variants of EMC virus, hybridoma techniques were applied. Balb/c mice immunized intraperitoneally every week with increasing doses of D variant of EMC virus beginning with $5.0{ imes}10^5$ PFU per mouse and ending with $1.0{ imes}10^7$ PFU per mouse for two months. Spleen cells from these immunized mice were hybridized with myeloma cells (NS-1). The hybrid cells were selected in HAT media. The antibody secreting cells among the hybrid cells were selected by viral neutralization test (against D variant of EMC virus) and then further cloned. Several different clones from these antibody-producing hybrid cells were characterized by immunological and biochemical methods. By these methods, it was found that some hybridoma produced monospecific antibodies which enabled us to identify specific antigenic variants.