DNAs encoding the mammalian histamine H4 receptors have been cloned and characterized. These recombinant molecules are capable of expressing biologically active histamine H4 receptor protein. The cDNA's have been expressed in recombinant host cells that produce active recombinant protein. The pharmacology of known histamine ligands is demonstrated. The recombinant protein may be purified from the recombinant host cells. In addition, recombinant host cells are utilized to establish methods to identify modulators of the receptor activity, and receptor modulators are identified.