The majority of viral meningitis cases is known to be due to ECHO virus 
infections on one hand, and mumps on the other. While the latter can be 
diagnosed by IgM antibody detection from one serum sample in the acute stage, 
diagnosis of enterovirus infections is by virus isolation and typing. An 
IgM-antibody test for ECHO 9 and 11 viruses is presented to evaluate the 
possibility of rapid serological diagnosis of ECHO virus meningitis cases. 36 
cases from five local outbreaks due to ECHO 6, 9, 11, and 30 viruses were 
characterized by virus isolation and serum neutralization tests. All sera (88 
samples) were assayed by a MACRIA (M-antibody capturing radioimmunoassay) to 
ECHO 9 and 11 viruses. While sera from all ECHO 9 and 11 cases, when taken at 
appropriate times, had IgM antibodies to the infecting type, a varying degree of 
cross-reactivity could be observed. Specificity problems are discussed in 
comparison with isolated cases of enteroviral infections due to different types, 
including Coxsackie B viruses.