Abstract An intergenic suppressor ( m ) of the amber ligase mutation ( amH 39 x ) was isolated and a number of possible mechanisms of suppression were investigated. The intergenic suppressor does not appear to involve suppression of the amber codon. Biological tests and enzyme assays indicated that the intergenic suppressor mutation m is not associated with a nuclease defect. Complementation studies revealed incomplete dominance of both alleles ( m or m + ) of the suppressor gene, suggesting a noncatalytic function for the wild-type suppressor gene ( m + ). In a mixed infection of E. coli B with an amber ligase mutant and the intergenic suppressed strain the former was preferentially excluded. The m mutants are rapid lysis mutants but they differ from the rII mutants in a number of properties.