We describe a simple plaque color assay for identifying oligonucleotide-directed mutations in cloned DNA fragments. The basis of the method is to: fuse the sequence of interest in-frame to the E.coli lacZ gene to produce a blue plaque phage, mutate the site of interest to a stop codon to generate a white plaque phage, and revert the stop codon and surrounding nucleotides to give a blue plaque phage containing one or more desired amino acid changes. The advantages of this cyclic method are that it produces truncated as well as amino acid substituted protein molecules, it can be repeated to introduce additional mutations, and it eliminates the need for labor intensive screening. Essentially any piece of DNA can be mutated using this method if the fragment has one open reading frame. If there is an open reading frame between the site and the lacZ gene, ATG codons can be inserted at the target site. We have used this method to produce termination and amino acid substitution mutants in the yeast CUP1 gene.