Salmonella choleraesuis strain 38 was repeatedly exposed to porcine neutrophils in vitro in an attempt to mimic conditions of the host and document changes associated with repeated porcine phagocyte exposure. After five passages through neutrophils, one strain designated 38 neutrophil adapted clone (38 PMNa-5X) was found to have increased resistance to phagocyte and hydrogen peroxide killing, loss of 50 kb virulence plasmid, a decrease in Vero cell invasion, and was less pathogenic in mice. Reintroduction of the 50 kb plasmid into strain 38 PMNa-5X restored invasiveness and partial virulence. No changes were noted in complement sensitivity or enzymatic activity;In a separate study, analysis of seventy-five pigs from four different field sources were examined for the presence of Salmonella. Feces, blood, buffy coat, and purified neutrophil samples were examined for the presence of Salmonella using buffered peptone and the Rappaport-Vassiliadis isolation procedure. Sixty-three percent of the animals were infected with Salmonella, 52% had S. choleraesuis isolated from purified neutrophil results indicate that neutrophils may contribute to the carrier state and should be cultured to identify S. choleraesuis infected animals;The mechanism of invasion used by virulent (strain 38) and avirulent (strain 9) S. choleraesuis was compared using a Vero cell invasion assay. Invasion was not affected by colchicine, but was significantly inhibited by cytochalasin B and D, chloroquine, and dansylcadaverine. This demonstrates the importance of microfilaments and receptor recycling in receptor-mediated endocytosis of Salmonella. Inhibition of endosome acidification did not affect the virulent strain as much as the avirulent strain and its ability to be recovered from the Vero cells. Outer membrane proteins (OMP) were extracted from both strains and compared on SDS-PAGE following surface protein labeling with [superscript]125Iodine. Virulent S. choleraesuis 38 had a unique 35 kDa protein. The OMP of both strains were examined by radioimmunoprecipitation and Western blot using guinea pig polyclonal antisera and the 35 kDa protein was again found to be unique to the virulent strain 38. Antisera against this 35 kDa protein significantly inhibited strain 38 invasion of Vero cells.