The chemosensitivity of leukemia cells, obtained from the peripheral blood of 35 patients with B-cell chronic lymphocytic leukemia, was determined by a leucine-incorporation assay in vitro. There was good correlation between the sensitivities to two purine analogs, 2-chlorodeoxyadenosine and 9-beta-D-arabinofuranosyl-2-fluoroadenine among previously untreated patients when tested at the 80% inhibition level. Previous exposure to chlorambucil did not affect the sensitivity to these compounds.