A specific hemagglutinin for chick erythrocytes was recovered for the first time from 2 recently isolated strains of WEE virus, but could not be demonstrated for a well-established laboratory-adapted strain. The hemagglutinin was no longer demonstrable after the freshly isolated virus had undergone more than 6 consecutive intracerebral passages in mice. The potency of hemagglutinin in the supernatant fluids of centrifuged brain suspensions increased with storage at 4 C, maximal titers requiring 1 to 7 days for development.The reaction between WEE hemagglutinin and erythrocytes from newly hatched chicks was maximal at pH 6.1, weak at pH 6.0, 6.2, and 6.3, and absent at pH 5.9 and 6.4 at room temperature. When the mixtures were reshaken at 10 and 30 minutes, a 2- to 4-fold increase in hemagglutinin titer resulted. While the receptors for this hemagglutinin were uniformly present on the erythrocytes of newborn chicks, they occurred irregularly on the erythrocytes of adult chickens and sheep. No receptor-destroying enzyme was associated with WEE hemagglutinin and the union between the hemagglutinin and red cells was much firmer than was the case with SLE, Jap B, and West Nile hemagglutinins, since resuspension of erythrocytes agglutinated by the WEE hemagglutinin did not reverse the reaction.WEE hemagglutinin was rapidly destroyed at pH 6.1, the pH for reaction with the erythrocyte, but not at pH 6.4 to 8.0. When hemagglutinin was stored at pH 8.0 at 4 C or -70 C no loss in potency was detected over a 7-month period.The normal serum inhibitor for the WEE hemagglutinin was absent from the protein fraction of human serum precipitated by acetone. In rabbit serum a small amount of normal inhibitor was still detectable in the acetone-precipitated fraction. Hemagglutination-inhibition tests with the acute and convalescent phase serum specimens from 7 patients who suffered from encephalitis due to western equine virus yielded distinct and specific diagnostic results.
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