Degrees of dominance of wild-type over amber alleles have been investigated for many genes of phage T4 by measuring progeny phage burst sizes of restrictive host cells mixedly infected with varying doses of each allele. The data presented demonstrate that whereas certain am mutants are almost completely recessive to wild-type others show considerable codominance with wild type. The results suggest that the expression of a single wild-type gene specifying an enzymatic protein is likely to be sufficient for a normal burst despite gene-product interactions. The expression of a single gene specifying a structural protein, on the other hand, appears generally not to be sufficient to yield a normal burst size. Applied to am mutants in genes controlling unknown functions involved in T4 morphogenesis, this criterion suggests that genes 4, 26, 28, 31, 38, 50, 51, and 65 are the most likely to code for gene products which function catalytically (if, in fact, any such functions exist).