To determine which clay minerals have the potential to bind swainsonine, an in vitro screening procedure was conducted. Twenty compounds were screened in one replicated in vitro trial. A commercially available bentonite bound approximately 10% swainsonine and was chosen for use in a subsequent lamb feeding trial. Twenty fine-wool lambs (30.5 +/- .7 kg) were assigned to one of five treatments (four lambs/treatment). Treatments included 1) positive control, 100% sorghum sudangrass hay, 2) 85% sorghum sudangrass:15% locoweed (Oxytropis sericea, 430 ppm [DM basis] of swainsonine) +0 g of bentonite, 3) Treatment 2 + 14 g of bentonite, 4) Treatment 2 + 28 g of bentonite, and 5) Treatment 2 + 42 g of bentonite. Lambs were fed the experimental diets for 35 d and were then fed the positive control diet for an additional 21 d. Lambs were weighted and blood was collected via jugular venipuncture weekly from d 0 through 56. On d 35, additional blood samples were collected 1, 2, 4, and 8 h after feeding. Weekly blood samples were analyzed for serum clinical chemistry profiles, and additional blood samples collected on d 35 were analyzed for serum metabolic hormones and serum swainsonine concentrations. Within 1 wk, serum alkaline phosphatase and glutamic oxaloacetic transaminase activities increased markedly (P < .05) in lambs fed locoweed. Serum insulin, growth hormone, and prolactin concentrations were not affected by feeding locoweed, but serum triiodothyronine and thyroxine concentrations were decreased by approximately 50% (P < .05) in lambs fed locoweed.(ABSTRACT TRUNCATED AT 250 WORDS)