Immunoglobulin M and immunoglobulin G (early and late) were separated by sucrose density gradient centrifugation from single inoculation hamster immune sera prepared against La Crosse, California encephalitis and snowshoe hare viruses. Fractionated antibodies were tested for differences in serological specificity by hemagglutination-inhibition, neutralization, and complement-fixation reactions. None of the immunoglobulins demonstrated a consistent pattern of superior specificity in serological reactions with closely related California group viruses. Results were most uniform in the neutralization test in that immunoglobulin M and early and late immunoglobulin G all showed similar cross-reactivities. By hemagglutination-inhibition and complement-fixation, a given immunoglobulin type from one serum would show good specificity while the same type from another serum would be quite cross-reactive. Immunoglobulin M neutralizing antibody fractions fixed complement as effectively as late immunoglobulin G neutralizing antibody fractions. Unlike results obtained by others with flavivirus immune sera, the immunoglobulin M fractions from California group immune sera were not more specific than immunoglobulin G fractions by neutralization, hemagglutination-inhibition, or complement-fixation tests. Single inoculation hamster antisera, prepared to California group viruses and collected late (28–30 days), were as specific in the neutralization test as antisera collected early (7–10 days).