In an investigation of a school outbreak of enterovirus-like illness and aseptic meningitis, an IgM antibody assay was used to identify persons with evidence of recent coxsackie virus B2 infection. During September and October 1989, 81 (25%) of 319 students and staff reported an enterovirus-like illness; of these, 17 (21%) also had aseptic meningitis. Attack rates for enterovirus-like illness were highest among varsity football team members (53%), and most of this illness occurred between 6 and 15 October. Coxsackie virus B2 was isolated from 2 varsity football players. IgM antibody studies confirmed that coxsackie virus B2 was the cause of the outbreak in the varsity football team and suggested it was not responsible for much of the disease in other students and staff: 63% of football team members were seropositive compared with 12% for other students in grades 6-12. This report illustrates the value of an IgM antibody assay in the investigation of enterovirus outbreaks to distinguish infection from illness.