Two genes, TEF1 and TEF2, encode the protein elongation factor EF-1 alpha in the yeast Saccharomyces cerevisiae. We have generated yeast haploid strains containing either TEF1 or TEF2 interrupted by insertion of a large piece of foreign DNA. Cells which contain either one functional copy of the EF-1 alpha genes are viable. In contrast, attempts to isolate a yeast haploid strain with both TEF1 and TEF2 inactivated have failed suggesting that the double gene disruption is a lethal event.