The following combinations were evaluated as cryoprotectors of human CFU-c enriched bone marrow frozen for one hour at -190 degrees C: 10% DMSO, 1% DMSO + 9% dextran (Dx), 1% DMSO + 9% HES, 15% glycerin (Gc), 1% Gc + 9% Dx, 1% Gc+ 9% HES, 5% Gc + 4% glucose. Cell counts and CFU-c content were compared with controls left at 4 degrees C. Highest values were observed with 10% DMSO and with 1% DMSO + 9% Dx. These studies confirm the superiority of DMSO to Gc as a cryoprotector. With the combination of 1% DMSO + 9% Dx the removal of DMSO prior to reinfusion of the marrow is obviated. This should facilitate the clinical use of autologous marrow transfusion.