In intestinal fluid samples from 39 human immunodeficiency virus type 1 (HIV-l)-infected patients, IgAandIgGlevelswereequivalent, whereasin10controls, IgAlevelsweresignificantly higher than those of IgG (P <.05). Intestinal IgA in patients contained predominantly monomeric IgAl, whereas IgAl and IgA2 subclass levels in controls were nearly equivalent and primarilypolymeric.The predominance ofIgGand monomeric IgA1inmucosalfluidsamples from HIV-I-infected patients suggests exudation of serum immunoglobulins into the intestine. The decreased proportion of mucosal plasma cells producing IgA and IgA2 in the HIV-l-infected patients(P <.01) may also contribute to the abnormal intestinal immunoglobulin levels. Intestinal IgG reacted withmost HIV-1antigens, whereasspecificIgAwaspresent inonly 10of 17 patients and reacted with only envelope (gp120 and gp160) and, less often, core (p17 and p24) antigens. Aberrant mucosal antibody responses and decreased integrity of the mucosal barrier may contribute to the intestinal dysfunction and infections that characterize HIV-1 infection. © 1994 The University of Chicago Press.