Abstract
Inactivation of large bacteriophages by u.-v. light is due to lethal mutations at a number of different loci. Each of these loci appears to be independently transferable from one phage particle to others inside the same bacterium. The transfer of loci between irradiated inactive particles results in formation of active phage if the infecting particles, taken as a group, possess at least one copy of each locus in active form. The number of loci can be calculated for each phage, and is at least 30 to 50 for some of them. Phage growth appears to take place by independent reproduction of each of these "unit loci" inside the host cell.