The objective of the present study was to evaluate the toxic effects of aflatoxin on some hematological parameters and to determine the preventive effectiveness of added glucomannan. In the study, 32 Merino rams were used, and the rams were separated equally to four groups as control (C), glucomannan (G), glucomannan+aflatoxin (AG), and aflatoxin (A). Erythrocyte, leukocyte count, hemoglobin, and hematocrit levels were decreased in A group compared with the other groups, and there was a reduction in similar parameters in AG group compared to control values. On the other hand, these parameters were tended to increase in AG group compared to A group values. Aflatoxicosis caused the lymphocytopenia and monocytopenia but increased percentage of neutrophil counts. In conclusion, the results determined in the study might be important to demonstrate the effects of aflatoxicosis and glucomannan on some haematological parameters before the clinical symptoms appear. © 2012 Nurcan Dónmez et al.