This serum cortisol assay is based on the competitive binding principal and is a solid phase enzyme immunoassay. The wells are coated with anticortisol IgG. The samples, controls or standards are incubated simultaneously with cortisol conjugated to the enzyme horseradish peroxidase. During incubation, the samples, controls or standards compete with enzymelabeled cortisol to bind anticortisol antibody immobilized on the wells. Unbound materials are removed by washing. Upon addition of chromogen substrate, color development will occur proportional to the amount of enzyme activity that in turn is inversely proportional to the amount of cortisol in the sample.