We previously proposed that the first enzyme for histidine biosynthesis in Salmonellatyphimurium plays a role in regulating expression of the histidine operon and that in order to play this role the enzyme must form a complex with histidyl-tRNA. Among the many observations that led to these conclusions were 1) that regulation of the histidine operon is defective in strains carrying a mutation in the gene for the first enzyme that renders the enzyme resistant to inhibition by histidine; and 2) that the enzyme purified from the wild type strain interacts specifically, and with high affinity, with histidyl-tRNA. The present study was carried out to test the prediction that the enzyme purified from the mutant strain described above would display a defect in its interaction with histidyl-tRNA. This prediction was fulfilled by the finding that purified histidine-insensitive enzyme does not bind histidyl-tRNA. Our results therefore suggest that the capacity of the enzyme to bind histidyl-tRNA invitro is a reflection of its regulatory function invivo.