Transgenic suspension cells of Oryza sativa L. cv. Dongjin utilized as a host for producing recombinant human cytotoxic T-lymphocyte antigen 4-immunoglobulin (hCTLA4Ig) were preserved in liquid nitrogen (- 196 degrees C) after slow prefreezing in a deep freezer (- 70 degrees C). The development of an optimal procedure for long-term storage was investigated by the addition of various concentrations of cryoprotectant mixture and osmoticum in preculture media before cooling. A pre-deep-freezing time of 120 min was the most effective for maintaining cell viability. Compared with mannitol, sorbitol, trehalose, and NaCl under the same osmotic conditions, 0.5 M sucrose was found to be the best osmoticum for preculture media. The cryoprotectant comprising sucrose, glycerol, and dimethylsulfoxide ( DMSO) was applied to the precultured cells, and a combination of 1 M sucrose, 1 M glycerol, and 1 M DMSO provided the best result. The viability with this optimized condition was 88% after cryocell-banking for 1 day. The expression of hCTLA4Ig in recovered callus from cryopreservation was also kept stable, and the production level was similar to that observed in noncryopreserved cultures.