A method is provided for detecting biomarkers in rare circulating cells by forming an enriched population of cells immunomagnetically followed by biomarker detection using binding compounds having releasable molecular tags. Preferably, biomarkers comprising one or more protein-protein complexes are detected in circulating cancer cells metastasized from a solid tumor. In the presence of biomarkers, the molecular tags of the binding compounds are released and separated from the assay mixture for detection and/or quantification.