.1. D. HERSHEY AND MARTHA CHASE 



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FIG. 3. Mottled plaques from bacteria infected with r and r+ virus. 



Experiments with Clumped Phage 



The following observations are pertinent to the proper interpretation of experi- 

 ments to be described in a later part of this paper. Since they are also of general 

 interest, we record them separately. 



The r mutants are unique among the known mutants of phage in that mixed 

 colonies of r and r"*" phage are easily recognizable as mottled plaques. Mottled 

 plaques are conveniently prepared by infecting bacteria with r and r"*" phage, and 

 plating before lysis so that the plaques originate not from single phage particles, 

 but from the mixed population liberated locally when the bacterium lyses (Fig. 3). 

 We have made use of this characteristic mottling as a test for the aggregation of 

 phage particles. 



Platings of mixtures of r and r"^ phage do not show mottled plaques (except 

 rarely by the overlapping of plaques), even after the mixture has been packed in a 

 centrifuge to allow every opportunity for the particles to stick together. Mottling 

 clumps can be produced, however, by agglutinating the mixture with antiserum. 



To prepare mottling clumps of phage, one adds to a mixture containing half r 

 and half r"^ phage, at a concentration of about 10" particles per ml, an amount of 



182 



