An enzyme-linked immunosorbent assay (ELISA), an immunofluorescence assay (IFA), a plaque-reduction neutralization (PRN) assay and an immunoblot assay, all by means of an antigen prepared from the attenuated Venezuelan equine encephalitis (VEE) vaccine strain of virus, were compared with the conventional haemagglutination-inhibition (HAI) assay for the serodiagnosis of VEE. The HAI assay, which includes the use of wild type virus antigen, was less sensitive than the other assays when known-positive samples of serum from an epidemic of VEE were tested. The superior sensitivity of the IgG ELISA was confirmed by assaying both VEE epidemic samples and a bank of samples from VEE vaccinees. Samples with antibody specific for other Alphaviruses, however, cross reacted weakly in this assay. The PRN, immunoblot and HAI assays, although less sensitive than the ELISA, proved more specific. Experimental infection of guinea-pigs demonstrated the value of the IgM ELISA in the early detection of VEE virus infection. Immunoglobulin M was first found at 4 days post-inoculation (p.i.) during the viraemic phase of infection. Immunoglobulin G was detected by ELISA, PRN assay and IFA at 6 days p.i. Immunoblot and HAI assays, however, did not give positive results until 10 days p.i. The results support the diagnostic use of ELISA for detecting VEE virus-specific IgM and IgG, and the use of the specific PRN assay for confirming the diagnosis.