The nature and the amount of polymeric IgA in sera from normal, adult humans was investigated by means of three markers that selectively differentiate polymeric IgA: the capacity to bind secretory component; the presence of the J chain; and the reactivity with an antiserum that recognizes only the polymeric form of IgA. The results demonstrated that the sera of healthy adults contain only minimal amounts (less than 1% of the total serum IgA) of the true dimeric IgA. These conditions are valid for healthy individuals only. Pathological conditions at the secretory sites may result in increased values for the dimeric as well as for the secretory IgA in serum. © 1975, SAGE Publications. All rights reserved.