Abstract: The invention relates to a method for determining a factor affecting a cell, which affects directly or indirectly the DNA, RNA and/or proteins of the cell or their synthesis machinery, in which recombinant DNA plasmid is transferred into the cell, and the initial point or points of the reproducing machinery responsible for its replication are subject to an adjustable promoter, which is controllable either by positive or negative feedback; the cell containing the recombinant DNA plasmid is brought into contact with the affecting factor; the affecting factor is allowed to affect the cell containing the recombinant DNA plasmid for a suitable time, after which the promoter adjusting the starting point of the reproducing machinery responsible for the replication of the recombinant DNA plasmid starts growing in the cell, unless the affecting factor has not inhibited the replication of the plasmid; the shift of the copy number of the recombinant DNA plasmid in the cell is determined directly or indirectly.