In this inhibition immunoassay undiluted serum reacts in solution with crude cellular CMV antigen in wells of microtestplates coated with hyperimmune CMV-reactive monkey IgG. CMV antibodies in the serum under test block (completely or partial) the fixation of antigen to the capture layer. Unblocked antigenic activity is in subsequent steps measured by the use of biotinylated CMV-reactive monkey IgG and peroxidase-conjugated avidin. The assay was evaluated in comparison with the CF test and was found superior both in terms of qualitative and quantitative detection of CMV antibodies. The results were uninfluenced by the presence in the sera of rheumatoid factor or autoantibodies (antinuclear antibodies). A characteristic feature of this inhibition immunoassay was the absence of equivocal results as demonstrated by analysis of 500 donor sera which were classified in two distinct separate groups: reactive and nonreactive. The assay is simple and reproducible and provides for a good reagent economy. Crude antigen can be used without sacrifice of specificity. Antigen from one Roux bottle proved sufficient for 25,000 duplicate tests.