Buffy coat cells were collected by leukapheresis from 14 patients with C.G.L. and one patient with M.M.M. Six of the C.G.L. patients had received busulphan at intervals varying between 401 and 38 d before leukapheresis. Significantly more CFUc's could be collected per leukapheresis in untreated patients and in those patients not treated within 105 d prior to leukapheresis. Survival of CFUc's following freezing was significantly less in the 3 most recently treated patients. These results suggest that in C.G.L. patients recently treated with busulphan, both the number of CFUc's in the peripheral blood and the ability of these cells to survive the freeze thaw process are reduced. However, sufficient CFUc's could theoretically be collected, stored and recovered post-thaw from one leukapheresis to allow complete haematopoietic restoration in all but 2 patients.