Starting with 7 ㎖ packed amoebae, mRNAs were purified by guanidium thiocyanate extraction followed by oligo(dT) column method. From the purified mRNA, cDNAs were synthesized and ligated with lambda expression vector. The library was immunoscreened by using monoclonal antibody against action of A. proteus. In a plaque-lifted disc containing 10,000 pfu, 50-75 plaques were found positive. Thus the validity of the cDNA library was proved. From one of the positive phage clones, plasmid clones containing action cDNA were obtained. The size of the insert was 1.1 kb having one each internal site for EcoRV and KpnI. It did not have any enzyme site for EcoRI, ApaI, NotI, PstI, XbaI, BamHI, PvuII, or XhoI. After Exo III/Mung bean nuclease deletion of the insert, deleted transformants were selected and partially sequenced.