9 beta-D-Arabinofuranosyladenine (adenine arabinoside, vidarabine, ara-A), is employed as an antiviral compound mainly against herpes virus infections. Toxicity of ara-A is of concern in clinical applications. This work reports quantitative changes of bone marrow hemopoietic progenitors in mice treated with ara-A. The experimental model is based on time survival curves following repeated intraperitoneal injections (200, 400 or 800 mg/kg twice a day for 4 days) of different doses of the drug. Our results show that ara-A causes damage to the hemopoietic progenitors. The induced damage is roughly proportional to the injected amount of the drug. Following termination of ara-A administration all tested populations rapidly recovered.