The present research work was performed on infusoria Spirostomum ambiguum. The microorganisms were placed in cuvettes with different habitats for determining different functional activity levels during for 24 hours. IrLE in the dosage 0,5 J/cm2 (exposure time 3,5 min, PFD 140 mW/cm2) increased infusoria's tolerance to metabo- lie intoxication in the bounded space with a nutrient medium. High IrLE doses and PED increasing caused tolerance depression. IrLE activating effect was observed under the substrate deficiency (in the distilled water) and initially low functional state of infusoria in the dosage 5,0 J/cm2 (exposure time 5,0 sec and PFD 1000 mW/cm2). The same dose with exposure time 35 sec and PFD 140 mW/cm2 had less activating effects. Photobiological effects of equal radiation doses depends on exposure time and power flux density as well as on the initial functional state of the tested cell.
