The scope of this study was to investigate possible relationships between He-Neon laser radiation and mitotic and phase indices in meristematic cells of Allium cepa L. bulbs. Our results indicate that mitotic index increased after irradiation depending this modification on the time exposure and the potency of the He-Neon beam. Phase indices were also modified: frequency of prophase increased, while inter- meta- and anaphase decreased: telophases remain unchanged. These variations were significative only when the preparations were irradiated a) with 5 mW for 10 min. or more, b) with 10 mW or c) when the preparations were processed 60 min. after irradiation. These findings could not be attributed to thermal changes. Modifications in RNA or protein synthesis could be responsible.