A preincubation of fission yeast cells with hyperosmotic solution improved the electro-transformation efficiency. The efficiency increased approximately five-fold when the cells were preincubated with 2.0 M sorbitol and 1.5 M NaCl at 30 degrees C for 60 min before an applied high electric pulse. Losses in the efficiency of the cells after hyperosmotic stress above 2.5 M sorbitol and 2.0 M NaCl were directly related to the marked reduction of viability. The efficiency at 2.0 M sorbitol gradually increased until 60 min of the preincubation period, but longer exposure resulted in a gradual decrease. On the other hand, when the cells of the osmotic-sensitive mutant were preincubated with isosmotic solution of 0.5 M sorbitol, the efficiency was also dramatically increased by approximately 15-fold. These improvements in efficiency were observed in sublethal conditions of osmotic stress regardless of osmoticums and strains.