The changes of optical transmission of the blood induced by the biostimulation laser (CLT-1106, X=830 nm) action were measured. The experiments have been made in vitro for two wavelength, 650 nm (red light) and 532 nm (green light). The different value of the hematocrit (50%, 10%, 2%, 0.25%) were applied.The transmission was measured with use of a integrating sphere. The range of the fluence rate changed from 500mW/cm2 to 100mW/cm2. Continuous wave and pulse of biostimulation laser were used. Influence of the biostimulation laser on the optical diffuse transmissionTd of blood was observed only for Ht=0.25%, and X=532 nm. The magnitude of changes was equal to a few percent related to the reference level, defined as a transmittance signal without laser irradiation. The observed changes in transmitted light can be explained by the aggregation and disaggregation processes and changes in the erythrocytes shape.