Recent microwave experiments have shown frequency dependent influences on the growth rate of bacteria. To determine whether microwaves are able to affect growth (or to induce lesions in cellular DNA of yeast cells), experiments were performed with millimeter microwaves at frequencies between 70 and 75 GHz. Saccharomyces cerevisiae cells were irradiated on millipore filter discs placed on agar plates in open petri dishes. A diploid strain of yeast (D5, Zimmerman), that is sensitive to genetic insult was used to study the effects of temperature and of microwave irradiation on cell survival, induction of mitotic recombination, and induction of cytoplasmic "petite" mutations. No evidence of altered survival, impaired function, or structural injury was seen at either frequency, even at power densities as high as 60 mW/cm2. Conventional heating had no deleterious effects until temperatures of specimens exceeded 50 degrees C. In addition, two haploid strains of yeast of opposite mating type were compared with respect to temperature and microwave treatment for formation of zygotes. The elevation of temperature due to the microwave treatment at 60 mW/cm2 and 2 mm distance was estimated to correspond to 3 degrees C.