The gamma(1)A present in saliva and colostrum exists largely in the form of higher polymers, the major component of which has a sedimentation coefficient of 11S. The 11S gamma(1)A in these fluids differs from the polymers found in normal and myeloma sera both immunologically and by the fact that their sedimentation coefficients are unaffected by disulfide bond reduction in the absence of urea. However, like other gamma-globulins the 11S gamma(1)A molecules consist of multiple polypeptide chains linked by disulfide bonds. Local synthesis of gamma(1)A in the salivary gland has been shown by fluorescent and autoradiographic studies, although the fraction of the total salivary gamma(1)A which is derived from local production is uncertain. No evidence of transport of intravenously administered I(131)-labeled 7S gamma(1)A from serum to saliva was obtained. Immunological specificity has been demonstrated in the salivary and colostral gamma(1)A. Whether that portion of the gamma(1)A which is immunologically specific is a piece incorporated during the local synthesis of gamma(1)A in the gland or is added by the epithelial cell in the process of transport remains to be determined. Antibody activity (isohemagglutinins) have been demonstrated in saliva and colostrum and have been shown to be of the gamma(1)A-type. In both of these fluids activity is associated primarily with gamma(1)A-polymers of 11S and 18S sizes. There appears to be an immunological system which is characteristic of certain external secretions. Its properties including the local production of a distinctive type of antibody separate it from the "systemic" system responsible for the production of circulating antibody. This system may play a significant role in the body's defense mechanisms against allergens and microorganisms.