Bacterial suspensions in a phosphate buffer were irradiated at wavelengths lambda of 632.8, 1066 and 1286 nm, incubated in Hottinguer broth for 60 min and assayed for viability by the standard surface-plating technique. The difference between the number of viable cells in the irradiated culture and the control was termed growth stimulation. Irradiation of the bacteria with an He-Ne laser (632.8 nm) or semiconductor lasers at 1066 and 1286 nm at various intensities and irradiation times produced two maxima in the growth stimulation vs. dose curve. The first maximum, in all cases, occurred near 50 J m-2, and the reciprocity law was obeyed. The second maximum occurred at an irradiation time of 100 s irrespective of the particular radiation intensity, and the reciprocity law was not obeyed. It is assumed that two different mechanisms are responsible for these two maxima in the growth stimulation vs. dose curve.