The authors have developed an ultra-sensitive system for antigens such as proteins and low-mol. weight drugs by utilizing the enormous amplification power of the polymerase chain reaction (PCR) during the signal-amplification step in immunoassays, especially those of the sandwich type. As a first model system, assays for interleukins 3 and 6, resp. were designed and compared with the very well established and highly sensitive ELISA assay. Depending on the detection system applied for the PCR products and exptl. conditions, the PCR-amplified assay is not only ∼1000 times more sensitive, but also allows the monitoring of concentrations down to a few fg/mL of sample and over a wider dynamic range of concentrations In addition the current project includes a comprehensive study on the detection and quantification of the DNA fragments (PCR products). The immuno-PCR method was developed to monitor small changes in the endogenous cytokine blood levels (interleukins 2 and 3, etc.) so as to monitor putative surrogate markers for the immunosuppressive activity of drugs. Nevertheless, there seems to be a variety of potential applications to detects trace amounts of drugs, rare metabolites, byproducts or contaminants. Endogenous plasma levels of interleukin 3 in humans were analyzed as a first application of the immuno-PCR assay system. Interleukins 3 and 5 might serve as surrogate markers for the efficacy of cyclosporin A on early and late reactions and associated development of bronchial hyperresponsiveness following allergen challenge.