We developed quantitative immunoenzymometric assays for human IgG and its subclasses by using monoclonal antibodies, an avidin-biotin detection system and, as the calibrant, the U.S. National Reference Preparation for Specific Human Proteins. The assays are sensitive (detecting as little as 6 micrograms/L), precise (average inter-assay CV less than 11%), and vary linearly with concentrations over a five- to 10-fold range, depending on the monoclonal antibody. We evaluated 22 different monoclonal antibodies, many of which remained highly reactive when immobilized in wells of microtiter plates coated with bovine serum albumin-glutaraldehyde to "capture" total IgG or subclasses of IgG in the sample. We demonstrated the specificity of the most reactive antibodies by using a panel of 20 purified myeloma proteins. The sum of IgG subclass concentrations correlated well (r = 0.84, p less than 0.001) with the total IgG measured in sera from 63 apparently healthy adults (26 men, 37 women). We estimated 95 percentile reference intervals for the immunoglobulins in these subjects and determined the following mean percentage distributions of IgG subclasses: IgG1 49, IgG2 33, IgG3 9, and IgG4 7. The availability of these assays should facilitate studies of the clinical significance of the subclasses.