In the paper, a new mathematical model for calculating the spectral characteristics of biological particles imitating formed elements of blood, as well as their aggregates has been put forward. The model takes into account the aggregate structure and multiple light scattering effect on them. The methods and algorithms based the T-matrix technique for calculating the laser radiation scattering on a biological cluster were considered. A particle size distribution function was determined on a basis of simulated in vitro experiment on light scattering by particle aggregates. A discussion of the obtained results was presented.