Mixed infection of nonpermissive bacteria by amber mutant and wild-type T4 phage reduces the burst size in a manner that depends on the nature of the gene product eliminated by the mutation. While the genotypic proportions produced by such mixedly infected cells were directly proportional to those in the starting mixtures, the fraction of the burst size compared to wild-type-infected cells was dependent on the kind of mutant used. For example, mutants in genes 56, 42, and 43, which control catalytic functions for DNA synthesis, produced normal burst sizes even at ratios of four mutant to one wild-type phage. The multiplicity of infection in these experiments was near 20, and most cells received one wild-type phage. By contrast, infection with mutants in tail fiber structural genes gave a normal burst size with 1:1 mixtures of wild-type phage, but showed a decrease in burst size below this ratio. The conclusions drawn are that enzymes of DNA replication are present in large excess over the amount needed for phage DNA synthesis, and that tail fiber structural proteins are made in a three- to fourfold excess over the amount needed for phage maturation under normal laboratory conditions.