Therapeutic applications of low—energy laser irradiation (LELI) of human patients suffering different diseases is currently experiencing fast progress in a number of countries including Russia (1] . This can hardly be clearly justified without efficient diagnostic technique which can provide objective and quantitative registration and monitoring of patients individual responses to the phototherapeutic treatment. This is particularly true in relation to the LELI of blood which is considered a very efficient way of treatment of a number of pathologies, including pulmanory, cardio-vascular, ischaemic, etc. A number of diagnostic techniques currently in clinical practice include, in addition to the conventional clinical blood tests, the measurements of refraction indeces and absorbance of whole blood and plasma which have been shown sensitive to LELI in vitro and in vivo at different wavelengths including 632.8 and 1234 nm [2,33, alternations in leycocyte metabolism under He-S-Ne 1 aser irradiation  , index of erythrocytes deformabi 1 ity (5], etc. We have tested the technique of laser backscattering nepheloznetry (LBN) which we have been developing for the diagnosis of blood pathologies [6,7] in respect to its sensitivity to laser irradiation of blood at in vitro and in vivo experimental conditions. The advantages of the LBN technique is that we deal with the whole blood but not with delute solutions or single blood cells and, also, this technique yeilds two characteristics of blood: deformability of erythrocyte which was previously shown sensitive to laser irradiation  and aggregation and disaggregation kinetics. In this paper we present preliminary results of our experiments showing the sensitivity of aggregation— disaggregation kinetics to LELI at certain experimental conditions and pathologies.