A plasmid-harbouring E. coli JM109 (3P) strain was cultivated for the overproduction of the genetically engineered fusion protein SPA::EcoRI. The fusion protein could be affinity bound selectively and directly from the 25-50% ammonium sulphate fraction of the lysate of E. coli JM109 on to IgG Sepharose. The preparation obtained thus exhibited high restriction activity on lambda DNA and linearized the plasmids pBR322 and pUC19. As compared to the native EcoRI the activity of the immobilized preparation was more resistant to thermal inactivation.