We describe the development and validation of interaction determination using unpurified proteins (IDUP), a method that selectively amplifies DNA sequences identifying ligand+target pairs from a mixture of DNA-linked small molecules and unpurified protein targets in cell lysates. By operating in cell lysates, IDUP preserves native post-translational modifications and interactions with endogenous binding partners, thereby enabling the study of difficult-to-purify targets and increasing the potential biological relevance of detected interactions compared with methods that require purified proteins. In 1DUP, target proteins.are associated with DNA oligonucleotide tags either non-covalently using a DNAlinked antibody or covalently using a SNAP-tag. Ligand target binding promotes hybridization of a self-priming hairpin that is extended by a DNA polymerase to create a DNA strand that contains sequences identifying both the target and its ligand. These sequences encoding ligand+target pairs are selectively amplified by PCR and revealed by high-throughput DNA sequencing. IDUP can respond to the effect of affinity-modulating adaptor proteins in cell lysates that would be absent in ligand screening or selection methods using a purified protein target. This capability was exemplified by the 100-fold amplification of DNA sequences encoding FRB+rapamycin or FKBP+rapamycin in samples overexpressing both FRB and FKBP (FRB.rapamycin+ FKBP, K-d approximate to 100 fM; FKBP.rapamycin+FRB, K-d = 12 nM). In contrast, these sequences were amplified 10-fold less efficiently in samples overexpressing either FRB or FKBP alone (rapamycin+FKBP, K-d = 0.2 nM; rapamcyin+FRB, K-d = 26 mu M). Finally, IDUP was used to process a model library of.DNA-linked small molecules and a model library of cell lysates expressing SNAP-target fusions combined in a single sample. In this libraryxlibrary experiment, IDUP resulted in enrichment of sequences corresponding to five known ligand+target pairs ranging in binding affinity from K-d = 0.2 nM to 3.2 mu M out of 67,858 possible combinations, with no false positive signals enriched to the same extent as that of any of the bona fide ligand+target pairs.