Whey acidic proteins (WAP) from the mouse, rat, rabbit, camel, and pig comprise two "four-disulfide core" domains. From a detailed analysis of all sequences containing this domain, we propose a new PROSITE motif ([KRHGVLN]-X-¿PF¿-X-[CF]-[PQSVLI]-X(9,19)-C-¿P¿-X-[DN]-X-¿N¿ -[CE]-X(5)-C-C) to accurately identify new four-disulfide core proteins. A consensus model for the WAP proteins is proposed, based on the human mucous proteinase inhibitor crystal structure. This article presents a detailed atomic model for the two-domain porcine WAP sequence by comparative modeling. Surface electrostatic potential calculations indicate that the second domain of the pig WAP model is similar to the functional human mucous proteinase inhibitor domains, whereas the first domain may be nonfunctional.