Synthetic tyrosine phosphopeptides were used to generate antibodies that recognize phosphotyrosine in the context of a defined sequence of flanking amino acids. Using phosphopeptide immunogens derived from regulatory or signal-generating motifs, "phosphorylation-directed antibodies" can be targeted to specific growth factor receptors or signal-generating proteins. In his paper, we show how phosphorylation-directed antibodies can be used in a colorimetric, high-throughput screen for drugs that modulate the function of specific growth factor receptors or signal-generating proteins.