The effects of ionizing radiation on the peripheral blood elements of primate Cebus apella were examined after a single 25.8 mC/kg (100 R) dose of whole-body X-ray. We determined the number of white blood cells, neutrophils, eosinophils, basophils, lymphocytes, monocytes and erythrocytes up to 90 days after exposure of 5 animals. Hemoglobin content and microhematocrit were also determined. Under these experimental conditions, the total number of leukocytes was reduced from 9440/mm3 to 5660/mm3 on day 6, mainly because of a decrease in the lymphocyte population from 5843.2/mm3 to 2349.0/mm3 on day 6. No significant differences were observed in the values of erythrocytes, neutrophils, eosinophils, basophils, monocytes, hemoglobin content and microhematocrit. All hematological parameters studied returned to normal values by day 90 after a single irradiation dose of 100 R.