Tanniferous temperate legumes are assumed to possess anthelmintic properties, but it is unclear whether this is the direct result of condensed tannins (CT) or is mediated indirectly via an improved metabolic protein supply. A metabolism experiment was conducted to differentiate between these factors by feeding the CT plant sainfoin (19.7% CP in DM) to lambs infected with the abomasal blood-sucking nematode Hemonchus contortus. A total of 18 infected lambs were fed sainfoin either untreated or treated with polyethylene glycol, a CT-inactivating agent, or a grass-clover mixture (13.2% CP in DM) over 3 wk (n = 6). Six uninfected lambs received the grass-clover mixture as a control. In addition to indicators of infection (fecal egg count, packed-cell volume, abomasal worm burden, and serum protein), nutrient digestibility, the balance of N and selected minerals, ruminal fluid characteristics, and plasma AA levels were determined mostly in the final experimental week. The specific effects of the sainfoin CT, the extra CP with sainfoin, and the infection were statistically evaluated by contrast analysis. The sainfoin CT exerted no beneficial effects on resilience to nematode infection and exerted only minor effects on ruminal ammonia or blood urea concentrations and the excretory pattern of N. Plasma alanine, aspartate, and proline concentrations tended to be greater (P < or = 0.09) because of the sainfoin CT, whereas the other AA remained unaffected. Intake of the mineral supplement was lower (P < 0.001) for lambs fed sainfoin compared with those fed sainfoin treated with polyethylene glycol. Feeding the high-protein sainfoin instead of the grass-clover mixture increased (P < 0.001) N retention and apparent OM digestibility, whereas digestibility of NDF and ADF were decreased (P < 0.001). Feeding sainfoin also decreased (P < or = 0.04) plasma alanine, glycine, isoleucine, and total nonessential AA compared with the grass-clover mixture. Although fecal egg count, worm burden, and packed cell volume were not affected by the greater CP supply associated with sainfoin feeding, serum albumin level was increased (P = 0.008). The lack of effects of sainfoin on resilience to nematode infection might have been the result of the unexpectedly low CT content (3.6% in DM) of the material used. It cannot be excluded that longer term feeding of this batch of sainfoin might have been effective.