Summary Thirty human bone marrow (BM) suspensions from patients with acute leukaemia patients in remission were processed with the Haemonetics 30 flow cell separator in order to separate buffy-coats and to treat them in vitro with a derivative of cyclophosphamide (ASTA Z 7557). After processing, the volumes of BM suspensions were reduced to 25%. Recoveries of leucocytes, CFUc and BFUe were respectively 62, 85 and 84%. In vitro treatment with doses of ASTA Z ranging from 50 to 140 μg/2× 107 leucocytes (according to the CFUc sensitivity of each patient) destroyed 95 ±5% of initial CFUc. After freezing and thawing, recovery of CFUc from treated BM was poor (24%) in comparison to that obtained with untreated BM (79%).