Many receptors stimulate proliferation of NIH 3T3 cells in a ligand dependent fashion. Based on this observation, we developed a high throughput assay of cloned receptor pharmacology. In this assay, receptors are transiently co-expressed with the marker enzyme beta-galactosidase. Receptors that induce cellular proliferation select and amplify the cells that also express the marker, thus the ability of ligands to alter receptor activity are reported as changes in enzyme activity. In the present study, we used this assay to evaluate the ability of agonist ligands to stimulate four cloned receptors. The agonists phenylephrine, carbachol, substance P and nerve growth factor selectively stimulated cells transfected with the alpha-1b adrenergic, m4 muscarinic, NK1 neurokinin and trkA neurotrophin receptors, respectively. These data demonstrate that a high throughput colorimetric assay performed in 96 well plates can be used to evaluate the pharmacology of ligands for cloned receptors belonging to a wide range of functional and pharmacological classes.