116 S. E. LURIA AND R. DULBECCO 



discussed above are in agreement with earlier observations (Luria and 

 Delbrugk 1942) on interference by a large excess of irradiated phage T2 with 

 the growth of active phage T2 when the inactive phage was mixed with 

 bacteria one minute and a half before the active one. 



Cross-reactivation between phage particles differing by one character 



Only one group of experiments will be discussed here, because of its bearing 

 on the analysis of the data presented in this article. Bacteria were infected with 



Table 7 



Cross-reactivation between one inactive particle of phage T2 and 

 one inactive particle of phage T2r 



Compare column (3) with column (1) 



(1) (2)* (3) 



FRACTION OF 



BACTERIA RE- OTHER BAC- 



CEIVING one TERIA THAT 



INACTIVE PAR- COULD GIVE 



MULTIPLIC- MULTIPLIC- _, 



EXPERI- DOSE OF TICLE T2 AND MOTTLED FRACTION 



ITY OF ITY OF . 



MENT RADIATION, one INACTIVE PLAQUES (AS OF MOTTLED 



INFECTION INFECTION „_ 



NO. HITS PARTICLE T2r FRACTION OF PLAQUES, 



for T2 for T2r 



AMONG THE THE BACTERIA FOUND 



BACTERIA THAT THAT LIBERATE 



LIBERATE AC- ACTIVE PHAGE), 

 TIVE PHAGE, CALCULATED 



CALCULATED 



* The values in this column include all bacteria with two or more particles of one type and 

 one or more of the other type, plus all bacteria with an active particle of one type and an in- 

 active particle of the other. The values are upper limits, since only a fraction of these bacteria 

 will actually give mottled plaques. 



phages T2 and T2r, both irradiated (r = 5 or 6). Low multiplicities were used, 

 so that a large proportion of the infected bacteria only received one inactive 

 particle, and, of those that received two, a great proportion received one 

 particle of each type. The infected bacteria were plated before lysis, and the 

 plaques examined for the proportion of "mottled plaques," that is, of plaques 

 containing both T2 and T2r active phages. Such plaques can only arise from 

 bacteria infected with both phages. It is seen from the data shown in table 7 

 that more than half the bacteria infected with one inactive particle of each of 

 the two phages actually liberate a mixture of active particles of both types. 

 This proves that recombination cannot result from reciprocal exchanges of 



289 



