PBV-1 and PBV-3 viruses isolated from mycelium of Penicillium brevi-compactum show similar morphology and practically the same size of virus particles. Both viruses have been shown to lyse very effectively E. coli C, K12, W3350, CR63. Reproduction of the viruses in E. coli CK cells is limited due to the phenomenon of modification by the host, and negative colonies formed by each on E. coli CK sheet differ in morphology. PBV-1 virus is highly infectious for E. coli B, while the capacity of PBV-3 to multiply in these cells is very low. The limited reproduction appears to be explained by modification of PBV-3 DNA in the E. coli B system. PBV-1 and PBV-3 viruses have been shown to differ serologically, although serological cross reactions indicate their antigenic relationship. The two viruses under study differ in the constituency and molecular weight of the main structural proteins: 36,000: 28,000: 26,500 and 17,000 daltons for PBV-1 and 39,000; 36,000; 32,500; 31,000 and 18,000 daltons for PBV-2.