Ribavirin (Virazole, 1-beta-D-ribofuranosyl-1,2,4-triazole-3-carboxamide), a broad-spectrum antiviral compound, was evaluated for effects on blood and bone marrow of rhesus monkeys when administered by intramuscular injection for 10 days in doses of 30 or 100 mg/kg/day (four monkeys/group). Both groups developed a normochromic, normocytic anemia that was mild in the low-dose group and severe in the high-dose group. A dose-related erythroid hypoplasia occurred during the treatment period. Myeloid precursors were not affected. Differential counts of erythroid precursors showed a significant decrease in late erythroid forms while early erythroid forms were either unchanged or increased. Megakaryocyte numbers were increased in both groups. Qualitative changes in marrow cells included vacuolization of erythroid precursors and of occasional white cell precursors and megakaryocytes, and the appearance of bone marrow histiocytes containing red cells in various stages of disintegration. Thrombocytosis occurred in both treatment groups, with platelet counts returning to control values after drug withdrawal. Platelet function was not affected by treatment. No drug-related changes were seen during the treatment period for total and differential leukocyte counts, mean corpuscular hemoglobin and mean corpuscular hemoglobin concentration. Reticulocyte counts and mean corpuscular volume increased after treatment then returned to control values. Osmotic fragility of erythrocytes was not changed. These data show that in monkey, ribavirin causes a dose-related decrease in circulating red blood cell mass that is due in part to suppression of late erythroid precursors in bone marrow. These effects are reversible when treatment is discontinued and are not predictive of potentially serious or lasting untoward effects of ribavirin.