The competence formation in 2 strains of Escherichia coli X7026 and Hfr H to isolated phage gamma DNA after the prolonged treatment of cells with Ca++ ions at low temperatures was investigated. In both strains studied the sensitivity of cells to phage lambda DNA increased during several days of maintenance at 4 degrees C in 0.2 M CaCl2, and reached the maximal value in 24-48 hours for E. coli Hfr H cells, and in 72-96 hours for E. coli X7026 cells. Cells maintained in CaCl2 for 24 hours and more interacted more effectively with DNA in the cold, and didn't need Ca++ ions at the last stage of transfection (incubation of the infectious mixture at 37 degrees C) as the freshly grown cells did. Variations induced in the cells after the prolonged action Ca++ ions were preserved only in the presence of CaCl2. After the washing of CaCl2 from the cells with 0.15 M NaCl they returned to the initial state. The competence formation in cells of E. coli X7026 under the effect of Ca++ ions was going on more actively when the cells were preliminary incubated for several days at 4 degrees C in the absence of CaCl2. E. coli Hfr H cells were resistant to this treatment.