Evidence of chromosomal damage was found in mitogen stimulated cultures of the lymphocytes from patients with chronic lymphocytic leukaemia (CLL), who had been treated with chlorambucil, if sensitivity of the cells to the drug could be demonstrated by an in vitro test (Lawler et al 1971). Additional chromosomal damage was induced when chlorambucil was added to such lymphocytes, in the presence of mitogens, in vitro. On the other hand, if the patients' lymphocytes had previously shown resistance to chlorambucil but little chromosomal damage resulted from therapy or could be induced when the drug was added in in vitro experiments.
Chromosomally rearranged lymphocytes found in patients with CLL, who have been treated with chlorambucil, may be interpreted as surviving cells with drug induced changes.