Previous studies implicated cysteine residues in the translational repressor (i.e. RNA binding) activity of the coat protein of bacteriophage MS2. It has been proposed that a protein sulfhydryl forms a transient covalent bond with an essential pyrimidine in the translational operator by a Michael addition reaction. We have utilized codon-directed mutagenesis methods to determine the importance of each of the two coat protein cysteines for repressor function in vivo. The results indicate that cys46 can be replaced by a variety of amino acids without loss of repressor function. Cys101, on the other hand, is more sensitive to substitution. Most position 101 substitutions inactivate the repressor, but one (arginine) results in normal repressor activity. Although the possibility of a transient covalent contact between cys101 and RNA is not categorically ruled out, construction of double mutants demonstrates that cysteines are not absolutely required for translational repression by coat protein.