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The purpose of this study was to determine the effect of He-Ne laser irradiation (632.8 nm, 10 mW) on the induction of acrosome reaction and mortality in bull sperm cells in comparison with two important capacitation agents; calcium and heparin. Frozen-thawed bull sperm cells were washed in percoll gradient and suspended at a concentration of 1 x 10(6) ml-1 in sp-TALP medium, capacitated in the presence of 2 mM CaCl2, 10 micrograms ml-1 heparin, or irradiated at fluences from 2 to 16 J cm-2, and incubated for 0, 30, 60 and 90 minutes. At the end of the incubation period, the percentage of sperm that were acrosome-reacted and dead was determined. The results obtained indicated that laser irradiation at all fluences produced a significant increase (p < 0.001) in the percentage of sperm cells that were acrosome reacted, and a significant decrease (p < 0.001) in the percentage of dead sperm at 90 minutes of incubation in comparison to other capacitation agents and the control group. The percentage of sperm cells with acrosome reaction was increased with increasing fluences of laser irradiation and time of incubation. It is conclude that the application of He-Ne laser irradiation at fluences from 2 to 16 J cm-2 induced the acrosome reaction and decreased the sperm mortality percentage in vitro of bull sperm cells.