A patient with moderate pancytopenia thought to be due to chloroquine ingestion was shown to have granulocyte progenitors (CFU-G) which were abnormally sensitive in vitro to chloroquine. As he required further anti-malarial prophylaxis, his marrow was prospectively tested in vitro with proguanil and its metabolite cycloguanil. The patient's CFU-G were also abnormally sensitive to these agents and predicted neutropenia was verified when the patient had a trial course of proguanil. We believe this is the first instance of in vivo confirmation of in vitro CFU-G drug sensitivities.