Plate II 

 Development of improved penicillia-producing strains in a selected culture of PenicilUum chrysogen um Thorn 



TOP: Parent strain, NRRL 1951, isolated from a cantaloupe at the Northern Regional Research Labora- 

 tory which yields penicillin up to lOOu/ml. in submerged culture. CENTER: A natural variant, NRRL 

 1951.B25, selected at the same Laboratory which yields up to 250u/ml. BOTTOM: An X-ray induced muta- 

 tion, X-1612, capable of yielding up to 500u/ml.; produced at the Carnegie Institution in Cold Spring Harbor 

 by irradiating conidia of NRRL 1951. B25. xt i 



Cultures on steep agar at 12 days. See discussion, p. 96. (Color photographs by Haines, Northern 

 Regional Research Laboratory. Reproduced through co-operation of Chas. Pfizer & Co., Inc.) 



