Experiments were conducted to evaluate whether infection of the respiratory tract of pregnant swine with pseudorabies (Pr) virus would induce the secretion of immunoglobulin A (IgA) antibodies in their milk as was observed after enteric infection with transmissible gastroenteritis (TGE) virus. The immune response of sows to Pr virus inoculated intranasally and to TGE virus inoculated orally/intranasally or via a natural infection was studied. Emphasis was placed upon titers and Ig classes of Pr and TGE virus-neutralizing antibodies in colostrum and milk. All animals exposed to Pr virus (alone or in combination with TGE virus) developed Pr-neutralizing antibody titers in both serum and milk. Pr antibody titers were generally higher in colostrum than in serum, but the opposite was true in milk compared with serum, with milk titers declining markedly during lactation. In contrast, TGE antibody titers in milk from experimentally or naturally infected sows usually remained higher than the corresponding serum titers and persisted at relatively constant levels throughout lactation. Gel filtration studies of milk indicated that the antibody activity against Pr virus was associated almost entirely with IgG fractions, with small amounts of antibody detectable in IgM fractions in colostrum from two of nine sows. By comparison, TGE antibodies were primarily of the IgA class, with varying but lesser amounts of antibody associated with the IgG class. Such results suggest that viral infection of the intestinal tract of the sow, but not the upper respiratory tract, stimulates the secretion of IgA antibodies in the milk.