The effects of laser light on the cellular DNA have not been extensively characterized. Low-power laser sources, such as the helium-neon (He-Ne) laser with a wavelength of 632.8 nm, have been found to produce photobiological and photodamage effects with evidence of interference with cell replication. We have investigated the effects of He-Ne laser irradiation on sister chromatid exchange (SCE) frequencies in sheep peripheral blood mononuclear cells (PBMC). Cultured cells were irradiated once at 6 selected energy intensities of laser irradiation and then stimulated with pokeweed mitogen and cultured in the presence of 5-bromodeoxyuridine (BrdUrd). The frequency of SCEs of both irradiated and non-irradiated cells were analyzed. The mean SCE of irradiated cells significantly increased with growing energy density up to a laser dose of 24 J/cm2, whereas after an energy density of 24 J/cm2, the SCE frequency decreased with increasing energy densities. We concluded that the application of He-Ne laser irradiation at energy densities ranging from 2 to 96 J/cm2 produced a different effect on SCE frequency in sheep PBMC in vitro.