Both haploid and diploid yeast cells are sensitive to the antibiotic G418. The former develop spontaneous resistance to G418 at a frequency of approximately 2 X 10(-6), however, the frequency of resistance in diploids is less than 1 X 10(-9) and was undetectable in these experiments. Mating of spontaneously resistant haploid cells to G418-sensitive strains yields sensitive diploids, indicating that spontaneous resistance to the antibiotic is a recessive trait. Both haploid and diploid cells can be efficiently transformed to G418 resistance by a plasmid carrying the Escherichia coli kanamycin resistance (kanr) marker. The ability to select for cells transformed with plasmids containing the kanr gene has facilitated the screening of a large series of temperature sensitive yeast mutants to determine whether any of them are allelic to RAD3.