Ninety-five pigs (initially 7.1 kg and 24 d of age) were used in a 28-d experiment to determine the effects of Ascophyllum nodosum seaweed extract (ANOD) on young pig growth performance and immune function in response to enteric disease challenge with Salmonella typhimurium (ST). Experimental treatments were arranged in a 2 x 4 factorial with main effects of disease challenge (control vs ST-challenge) and dietary addition of ANOD (0, 0.5, 1.0, and 2.0% of diet). Pigs were fed ANOD diets for 14 d and then challenged orally with ST or sterile media. There were no main effects of ANOD on growth performance end points, although there were significant quadratic effects of ANOD on ADG (P < 0.04) and final weight (P < 0.003), both being greatest at 1.0% ANOD. There was a positive linear effect of ANOD inclusion on ADFI (P < 0.07) and a negative linear effect on the gain-to-feed ratio (G/F) (P < 0.05). ST-challenge reduced ADG (P < 0.05), ADFI (P < 0.05), and G/F (P < 0.05) in the first week following challenge. Daily estimates revealed reductions in feed intake in ST-infected pigs on d 2 to 4 following infection (P < 0.05). Rectal temperature was increased maximally 2 d following ST-infection (P < 0.05). A disease challenge x time interaction (P < 0.001) was observed for serum haptoglobin and alpha1-acid glycoprotein. Serum immunoglobulin M (IgM) was not influenced by disease challenge, but IgM declined (P < 0.001) in all pigs over time. Serum immunoglobulin G (IgG) also was not influenced by disease challenge, but IgG tended (P < 0.08) to increase over time. In vitro culture of porcine alveolar macrophages with 10 mg/mL ANOD elevated (P < 0.05) prostaglandin E2 (PGE2) production over that of controls at 3 and 24 h of culture. There was no interleukin-10 response by porcine splenocytes cultured in vitro with 0.005, 0.05, 0.5, or 5 mg/mL ANOD. We conclude that this model of enteric disease elicits an acute phase response that is accompanied by increased rectal temperature and diminished feed intake. Furthermore, our results indicate some beneficial effects of dietary ANOD on growth performance and no influence of dietary ANOD on immune response in the presence or absence of ST-challenge. However, high ANOD concentrations are capable of activating porcine alveolar macrophages in vitro to secrete PGE2.