A rapid and simple protocol for the transformation of plasmid DNA using CaCl₂ is reported. 3～10 min incubation of cells with DNA on ice and spreading onto agar plates pre-warmed to 37℃, resulting in more than 10⁵ colonies/μg plasmid DNA. The effect of Ca²⁺ concentration and store time at 4℃ on transformation efficiency is also discussed.