108 BENZER ET AL. 



the sensitive bacterium (Hershey, 1948). In this case, the presence 

 of the antibody interferes with some step in the muhiphcation of the 

 virus subsequent to its attachment to the bacterial cell. Bacteriophages 

 partially coated with antibody which did not attach to crucial spots 

 not only adsorb but also reproduce (Burnet et al., 1937). To explain 

 these results it has been proposed (Burnet, 1937) that there are two 

 kinds of receptors on the phages: one for the antibody and one for the 

 bacterial surface. 



When a phage is adsorbed on the sensitive host, subsequent treat- 

 ment with antiserum no longer interferes with its multiplication (Del- 

 briick, 1945a). The latent period and the burst size are unimpaired. 

 This fact has proved to be very useful in permitting a precise estimate 

 of the amount of free and adsorbed phage. If the unadsorbed phage 

 of an adsorption mixture has been neutralized by addition of anti- 

 phage return, the ratio of the plaque count found after addition of the 

 serum to that present initially is then the fraction of t}ie infective 

 centers due to adsorbed phage. 



Experiments by Burnet have shown that formalin-killed bacteria 

 heavily coated with phage are agglutinated by the homologous anti- 

 phage serum but are not agglutinated by heterologous serum. This 

 indicates that the phage is still able to react with antiserum when ad- 

 sorbed to a killed bacterium. 



8. Virus mutants. — All mutations to be considered are spontane- 

 ous mutations, no reliable data on induced mutations are available. All 

 mutations to be considered occur (presumably) during multiplication 

 of the phages, not in stored stocks. 



The important mutants are: 



(1) Host range mutants, designated by the letter h (Luria, 1945; 

 Hershey, 1946a). 



(2) A class of plaque-type mutants, giving a larger plaque than 

 the wild type, with a clear instead of a turbid halo. This type is desig- 

 nated by the letter r (Hershey, 1946a, b). There are many genetically 

 distinct, but phenotypically similar mutants of the r type, and these 

 are distinguished when necessary by numerals following the r (Her- 

 shey and Rotman, 1948) , 



Thus: T2 means the wild type of this strain. 



T2h means its host range mutant, in most cases a mutant that 

 can multiply on B/2 (cf. glossary). 



