Ultraviolet (UV) irradiation has high potential to inactivate a wide range of biologic agents and is one of several nonadditive technologies being studied. The photoinactivation property of pulsed UV laser radiation (at wavelengths of 355 and 266 nm), used as an effective physical means to inactivate two typical microorganisms, prokaryotic (Escherichia coli K12) and eukaryotic (Saccharomyces cerevisiae), with respect to dose and exposure times, was examined. An E. coli population of 1.6 x 10(4)colony-forming units (CFU)/ml was inactivated with a dose of 16.7 J/cm(2) energy at 355-nm wavelength. However, E. coli cells at higher concentrations were inactivated by only 98% using the same dose. Interestingly, an E. coli population of 2 x 10(7) CFU/ml was completely inactivated using only 0.42 J/cm(2) at 266-nm wavelength (P <= 0.05). With respect to S. cerevisiae, the results were similar to those of E. coli irradiation considering that S. cerevisiae is 100 times larger than E. coli. A dose of 16.7 J/cm(2) completely inactivated an S. cerevisiae population of 6 x 10(3) CFU/ml at 355-nm wavelength. Exposure to 266-nm wavelength, with energy doses of 1.67, 0.835, and 0.167 J/cm(2), successfully inactivated S. cerevisiae populations of 3 x 10(6), 1.4 x 10(5), and 1.5 x 10(4) CFU/ml, respectively (P <= 0.05). In conclusion, compared with 355-nm wavelength, a pulsed UV laser at 266-nm wavelength inactivated a high titer of bacterial and yeast indicator standards suspended in phosphate-buffered saline-A.