Human cord blood mononuclear cells (MNCs) purified by Ficoll-Hypaque gradient centrifugation were cryopreserved in a vitreous state. The vitrification solution was MEM-alpha medium containing 4-8 mol/l (M) ethylene glycol (EG). A pellet of 1.5-2.0 x 10(6) MNCs was added with 10 microliters of 1 M EG and then 50 microliters of high concentration EG at room temperature (the final concentration was 4-8 M EG). The cell suspension was directly plunged into liquid nitrogen, stored for various periods and rapidly warmed in a water bath at 37 degrees C. The vitrification solution was removed by addition of MEM-alpha medium containing 1 M sucrose. The present experiments demonstrated that a vitrification solution consisting of 8 M EG produced the highest recovery rate for MNCs (89.5 +/- 8.5%), CFU-GM (66.6 +/- 20.8%) and BFU-E (66.5 +/- 22.8%) and the highest Trypan blue viability (98.7 +/- 0.4%). This ultrarapid cryopreservation method may be useful for the preservation of hematopoietic progenitor cells.