A study was conducted to determine the effect of feeding yeast cell wall (YCW) products on the physiological and acute phase responses of crossbred, newly-received feedlot heifers to an endotoxin challenge. Heifers (n ¼ 24; 219 2.4 kg) were separated into treatment groups receiving either a control diet (n ¼ 8), YCW-A (2.5 g/heifer/d; n ¼ 8) or YCW-C (2.5 g/heifer/d; n ¼ 8) and were fed for 52 d. On d 37 heifers were challenged i.v. with LPS (0.5 mg/kg body mass) and blood samples were collected from 2 h to 8 h and again at 24 h relative to LPS challenge.