Busulfan is an alkylating chemical used as an antineoplastic agent, with selective action confined to myelosuppression. The drug reacts with the N-7 position of guanosine and interferes with DNA replication and transcription of RNA. Both mice and rats evidenced the full spectrum of developmental toxicity when administered busulfan by the intraperitoneal route on various days of the organogenesis period to gravid animals. The recommended human therapeutic dose for busulfan is 0.12 to 4 mg/kg/day po. It should be emphasized that no specific pattern of malformation is obvious among the recorded cases, although toxicity is apparent with use of this drug, and it is generally considered potentially teratogenic. Intrauterine growth retardation was observed in most of the recorded cases; one investigator stated that 40% of infants resulting from maternal treatment with antineoplastic agents, including busulfan, were of low birth weight. Busulfan is a smaller, slightly hydrophilic compound consisting of two polar functional groups separated by a four-carbon scaffold.