A lab technique used mainly for identifying and selecting mutant colonies of a microorganism when the mutant can only be selected against. The microorganisms are cultured on a plate of agar, then a replica is made by blotting the colonies with a membrane and blotting the membrane on another plate. This results in all colonies from the first plate being in identical locations on the second plate. The second plate either contains some substance which kills off the mutation of interest, or lacks a substance necessary for the mutant to grow. Later, it is determined which colonies on the first plate failed to grow on the second plate.