1.Studies were made of 487 sera for antibodies to the VEE virus both by the hemagglutination-inhibition (HI) and the neutralization (N) tests.
2.Of 344 sera that were negative in the HI test all were negative in the N test. Of 100 sera that exhibited titers of 1:80 or higher in the HI test all neutralized the virus. Among 43 sera with HI titers of <1:10 to 1:40 there were 13 that failed to neutralize the virus. Tests on these 13 sera indicated that another group A virus could have been responsible for the HI results, and in nine instances it was established that the Mayaro or another virus closely related to it might have been responsible for the production of the VEE HI antibodies of low titer.