Two methods for cryopreservation of bone marrow stem cells were compared using bone marrow obtained from 36 patients. Included in this group were 21 persons with the diagnosis of leukaemia including 14 either with acute myeloid or lymphoblastic leukaemia in remission following intensive remission induction chemotherapy. After freeze-preservation and reconstitution, all marrow samples were tested for nucleated cell (NC) recovery and grown on agar to assess colony forming units (CFUC) and cluster forming units in culture (CluFUc). A slow dilution reconstitution method using freezing media containing AB negative plasma resulted in recovery of 85% of the CFUc activity of fresh marrow. This result was significantly better than the 47% CFUc recovery obtained when freezing media without plasma and a rapid dilution reconstitution technique were used. NC recoveries following slow dilution (51%) and rapid dilution (44%) were not significantly different. CluFUc were disproportionately reduced compared with CFUc although yielding similar results with both methods (26% and 32%). No correlation was found for either method between CFUc and NC recovery or between CFUc and CluFUc recovery in cryopreserved bone marrow.