Abstract: The invention provides a method for determining the activation status of receptor tyrosine kinase (RTK) pathways in either cell samples or patient samples by measuring receptor dimerization and relative amounts of protein-protein complexes or activated effector proteins that are characteristic of an RTK pathway. The invention also provides a method of using such status information to select patients responsive to pathway-specific drugs, and more particularly, to methods for measuring ErbB receptors and receptor complexes and using such information to select patients responsive to ErbB pathway-specific drugs. Preferably, methods of the invention are implemented by using sets of binding compounds having releasable molecular tags that are specific for multiple components of one or more complexes formed in RTK activation. After binding, molecular tags are released and separated from the assay mixture for analysis.