Human immunoglobulin G subclass 4 (IgG4) is the least abundant IgG subclass in serum and is frequently induced by repeated allergen exposure. IgG4 does not have allotype variation and does not interact with complement component C1q. Clinically elevated serum IgG4 levels occur in a variety of conditions including the multi-organ fibro-inflammatory IgG4-related disease. The sensitive quantitative measurement of total human IgG4 antigen in serum, plasma, hybridoma cell supernatants, ascites or other biological fluid samples is easily performed with this 96 well strip format ELISA kit. The assay does not detect immunoglobulins of other classes or subclasses. The mean value of IgG4 in serum of healthy donors was found to be 1.04 mg/ml. The assay measures human IgG4 in the 0.5-500 ng/ml range. Samples giving human IgG4 levels above 500 ng/ml should be diluted in blocking buffer before use. A 1:200,000-1:100,000 dilution for plasma and serum is suggested for best results. Human IgG4 in samples will bind to the biotinylated capture antibody which is bound to the avidin on the plate. After appropriate washing steps, horseradish peroxidase labeled polyclonal anti-human IgG antibody binds to the captured protein. Excess antibody is washed away and TMB substrate is used for color development at 450nm. A standard calibration curve is prepared along with the samples to be measured using dilutions of human IgG4. Color development is proportional to the concentration of IgG4 in the samples. All reagents and standards are provided in these ELISA kits.