The formation and repressibility of the arginine biosyntietic enzymes acetylornithine delta-aminotransferase (EC 2.6.1.11), acetylornithine deacetylase (EC 3.5.1.16), ornithine carbamoyltransferase (EC 2.1.3.3), and argininosuccinate lyase (EC 4.3.2.1) were studied in an Escherichia coli W derivative (strain 250-10) that carries (a) a mutant allele of the argR regulatory gene causing a diminished repression-derepression range and (b) a streptomycin resistance mutation. In comparison with the streptomycin-sensitive parent 250, all four enzymes (a) are formed as smaller proportions of the total protein (overall range, 12% to 71%), whether the conditions are repressive (arginine excess) or derepressive (arginine restriction), and (b) show increased repressibility ratios, the carbamoyltransferase giving the largest increase (from 5.7 to 25.0). These effects appear to depend on the concurrent expression of the regulatory-gene and streptomycin resistance mutations, as indicated by analogous experiments with canavanine-resistant mutants of 250-10 that have partial argR- character. The results provide evidence for translational repression in the arginine system, and are interpreted in terms of a functional interaction of a mutant arginine repressor with a mutant S12 ribosomal protein. The locale of translational repression may be near the site of S12, and this mode of regulation may involve initiational selectivity of groupwise recognizable arginine messenger RNA's.