The percentage of pre-B cells, mature B cells and IgM plasma cells were reduced in the marrow of children receiving continuous cytotoxic drug treatment to maintain leukaemia remission, compared with children receiving intermittent drug treatment in the UKALL V trial or untreated controls. When treatment was ended, the proportion of marrow pre-B cells rose above that of the controls and remained elevated for more than 6 months. These observations define more precisely the cellular basis of suppression of antibody immunity by cytotoxic drugs. They also suggest the existence of a complex feedback control of pre-B cell numbers and B cell differentiation during recovery.