(a) contacting a polypeptide comprising the amino acid sequence defined by SEQ ID NO: 2 or a histamine binding fragment thereof in the presence of a known amount of labeled histamine or a surrogate thereof with a sample to be tested for the presence of a histamine agonist or antagonist;

(b) measuring the amount of labeled histamine or surrogate thereof specifically bound to the polypeptide;

(c) contacting cells expressing a polypeptide comprising the amino acid sequence defined by SEQ ID NO: 2 or a histamine binding fragment thereof with said sample; and

(d) measuring at least one cellular function modulated by the binding of a ligand to a polypeptide comprising the amino acid sequence defined by SEQ ID NO: 2;

whereby a histamine agonist in the sample is identified by measuring:

(i) substantially reduced binding of the labeled histamine or surrogate thereof to the polypeptide compared to what would be measured in the absence of said sample in step (b); and

(ii) substantially increased level of said cellular function compared to what would be measured in the absence of said sample in step (d);

and whereby a histamine antagonist in the sample is identified by measuring:

(i) substantially reduced binding of the labeled histamine or surrogate thereof to the polypeptide compared to what would be measured in the absence of said sample in step (b); and

(ii) substantially similar or reduced level of said cellular function compared to what would be measured in the absence of said sample in step (d).