An immunoglobulin M (IgM) capture enzyme immunoassay technique was adapted for the detection of antibody to Colorado tick fever virus in sera from 84 individuals for whom diagnosis had been confirmed by virus isolation or neutralization test. Titers were compared with those for IgG and neutralizing antibodies in these Colorado tick fever cases. IgM antibody titers were higher than neutralizing antibody titers, but neither appeared until 1 to 2 weeks after the onset of illness. Neutralizing antibodies were detected earlier than IgM antibodies, and both were detected with greater frequency than IgG antibodies. Late-convalescent-phase sera contained both neutralizing and IgG antibodies, but IgM was all but undetectable by 2 months after onset. Although the neutralization test may remain the serological test of choice, the enzyme immunoassay for IgM antibody offers a simple and more rapid method of serodiagnosis; the enzyme immunoassay is, however, less sensitive than the neutralization test. Furthermore, because there was a sharp decline in IgM antibody after 45 days, the presence of IgM antibody in a single serum sample provides a basis for the presumptive serodiagnosis of recent Colorado tick fever virus infection.