BACKGROUND: Bone marrow is extremely vulnerable to damage caused by radiation therapy. Hence, bone marrow suppression is an important side effect of radiotherapy. Effective use of radiotherapy is therefore compromised by radiation-related injuries. MATERIAL AND METHODS:SixGuinea-pigs were recruited for the study of which three were subjected to total body irradiation with Co60while the other three served as controls. Bone marrow and peripheral blood samples were collected before and at days 9, 14 and 21, post irradiation. Manual and automated counts were performed for bone marrow nucleated cells and peripheral blood cells respectively. RESULTS: Declining bone marrow cellularity was evident immediately post irradiation. Mean ± SD of marrow cell counted per mm3 were 121,924±281, 87,603±772, 121,367±375 and 122,750±1000 pre-irradiation and days 9, 14 and 21, post-irradiation (p-values 0.10, 0.27 and 0.29 respectively). Significant drops in counts were noticed on day 9 post-irradiation for all red cell parameters (p-values <0.05), for Total White Blood Cell Count and Neutrophil count (p-values <0.05) and also on days 14 and 21 for Lymphocytes (p-values <0.05) and on day 21 for Eosinophil/Basophil/Monocytes (p-value <0.05). A significant drop in platelets counts was also noticed on day 9 (p-value <0.05) which significantly increased above pre-irradiation value on day 21. CONCLUSION: Total body irrradiation with Co60significantly affects the bone marrow with maximum reductions in marrow nucleated cells and peripheral blood cells counts on day 9 post irradiation. KEYWORDS: Ionizing, Radiation, Marrow, Animal, Model