Bone marrow granulocyte progenitor cells (CFU-C) were assayed in methyl-cellulose prior to cryopreservation in Dimethylsulfoxide (DMSO) and after thawing and diluting the DMSO. The time of dilution from 10% to 1% DMSO and the temperature of the sample and diluting media were studied. Compared with samples diluted at 0 degrees-4 degrees C, samples which were diluted at 24 degrees C were more viable by Trypan Blue exclusion (p less than .01) and had greater CFU-C growth in vitro (p less than .01). There was no advantage to prolonging dilution time from 10 minutes at a constant rate to 40 minutes using stepwise technique. Recovery of CFU-C at 24 degrees C ranged from 40% to 114% with a mean +/- S.D. of 67% +/- 19.5%. There was evidence that clonogenic cells were selectively preserved under the conditions described.