Thirty-two crossbred (Suffolk x Rambouillet) wether lambs were used to examine the effectiveness of protein source (soybean meal [SBM] or fish meal [FM]) in alleviating decreased performance associated with dietary aflatoxin (AF) in growing lambs. After a 21-d adaptation period to concentrate diets, lambs were assigned to the following dietary treatments: 1) SBM, 0 mg of AF; 2) FM, 0 mg of AF; 3) SBM + 2.5 mg of AF/kg diet; or 4) FM+2.5 mg of AF/kg diet (two lambs/pen; four pens/treatment). Diets were fed 35 d, at which time AF was removed from the diet (except one pen/protein source) and lambs continued on study for an additional 32 d. On d 67, all lambs were killed and necropsied. Average daily gain, feed intake, and gain/feed were similar (P > .10) among lambs fed SBM or FM; however, lambs fed AF had lower (P < .01) feed intakes, daily gain, and gain/feed. Feed intake remained lower (P < .01) after AF was removed from the diet. Aflatoxin elevated (P < .01) aspartate amino transferase and gamma-glutamyl transferase activities and total protein and cholesterol concentrations while decreasing (P < .05) alkaline phosphatase, glucose, cholinesterase, albumin, inorganic phosphorus, iron, and total-iron-binding capacity. Hematocrit, white blood cell count, and prothrombin time increased (P < .01) in lambs fed AF. No AF or protein effects were seen on ruminal VFA, pH, or lymphocyte blastogenesis (P > .10).(ABSTRACT TRUNCATED AT 250 WORDS)