Fifteen Holstein bull calves weighing 135-160 kg (300-350 lb) were randomly assigned to 3 groups of 5 animals each. Each calf was individually fed free choice Bermuda grass hay and 3.63 kg (8 lb) of a commercial 14% crude protein ration/day. Group 1 (control) received no Cassia obtusifolia seed, group 2 (low exposure) received 0.45 kg (1 lb) of C obtusifolia seed (12.5% of ration and approximately 0.27% of body weight) and group 3 (high exposure) received 1.81 kg (4 lb) C obtusifolia seed (50% of ration and approximately 1.19% of body weight) from day 0-9, and then received 1.36 kg (3 lb) (37.5% of ration and 0.89% of body weight) from day 10-24. The effects of low and high C obtusifolia seed contamination were intermittent feed refusal, diarrhea, and decreased weight gain. Average daily weight gains (ADG) for the control, low exposure and high exposure groups, were 0.76, 0.53, 0.31 kg/day, respectively. The ADG of the control calves was significantly greater than the ADG of the high exposure calves (p less than 0.0001) and the low exposure calves (p less than 0.0059). The ADG of the low exposure calves was significantly greater than the ADG of the high exposure calves (p less than 0.0209). No change was observed in serum aspartate aminotransferase, creatine phosphokinase, lactic dehydrogenase activity, and histologic examination showed no evidence of muscle necrosis.